An Example of Phylum Mollusca: Neopilina

In this article we will discuss about of Neopilina:- 1. Discovery of Neopilina 2. Habit and Habitat of of Neopilina 3. External Structure 4. Coelom 5. Digestive System 6. Locomotion 7. Respiratory System 8. Circulatory System 9. Excretory System 10. Nervous System and Sense Organs 11. Reproductive System 12. Relationships with other Molluscs.

Contents:

1. Discovery of Neopilina
2. Habit and Habitat of of Neopilina
3. External Structure of Neopilina
4. Coelom of Neopilina
5. Digestive System of Neopilina
6. Locomotion of Neopilina
7. Respiratory System of Neopilina
8. Circulatory System of Neopilina
9. Excretory System of Neopilina
10. Nervous System and Sense Organs of Neopilina
11. Reproductive System of Neopilina
12. Relationships of Neopilina with other Molluscs

1. Discovery of Neopilina:

Neopilina galatheae is a living representa­tive of the class Monoplacophora. This newly discovered species possesses peculiar ad­mixture of molluscan and annelidan fea­tures. Neopilina is a very primitive member amongst the molluscs and represents a sort of connecting bridge between the annelids and molluscs.

There are many fossil relatives of this genus which were quite abundant in Cambrian to Devonian strata. The genus, Neopilina, was collected from the western coast of Mexico. This discovery has added new dimension as regards the phylogenetic relationship of the molluscs as a whole with the annelids.

The Danish Zoologist, Lemche first collected the species in 1952 from the assorted molluscs collected in Galathea ex­pedition. Another species was discovered during the voyage of American Research Vessel, Vema in 1958. The anatomy of Neopilina galatheae was exhaustively worked out by Lemche and Wingstrand (1959).

2. Habit and Habitat of of Neopilina:

Neopilina is a deep-sea variety and was collected from the Pacific Ocean at depths from 2500 to 5000 m. Neopilina lives in mud and feeds mainly on foraminifera. It also takes radiolarians and diatoms as evidenced by their remains in its stomach.

3. External Structures of Neopilina:

The body of Neopilina is more or less bilaterally symmetrical and exhibits metameric segmentations like annelids. Neopilina galatheae is about 3.7 cm long, 3.3 cm wide and 1.4 cm high. The dorsal side of the body is covered by a thin shell. The shell is circu­lar in outline. The apex of the shell is drawn anteriorly and is slightly bent (Fig. 16.2A).

The shell is composed of a thick calcareous middle prismatic and innermost calcareous nacreous layer, covered over by periostracum. These three layers of the shell are formed from the margin of the mantle.

The soft parts of the body are seen only from the ventral side. There is a large flat foot like that of Chiton. Surrounding the foot and head there is a deep pallial groove contain­ing five pairs of gills.

The mouth is situated medially in front of the foot and the anus is located posterior to the foot (Fig. 16.2B). In front of the mouth there is a transverse labial swelling which prolongs laterally into two ciliated lobes, one on each side.

These ciliated lobes are comparable to the anterior labial palps of Unio. Anterior to the transverse swelling there is a pair of small pre-oral ten­tacles. There is a swollen labium which bears a number of postoral tentacles on the anterior surface.

There are eight pairs of dorso-ventral muscles in Neopilina (Fig. 16.3A). These muscles originate from the shell and extend to the median wall of the pallial groove. The dorso-ventral muscles correspond to the columellar muscles of the gastropods.

4. Coelom of Neopilina:

The coelom is represented by a caudal pericardium and two pairs of spacious gonocoels.

5. Digestive System of Neopilina:

The mouth leads into the buccal cavity which is covered by a cuticular plate. The pharynx extends to the dorsal side and bears a radular sac with a radula.

The pharynx pro­duces a pair of long, thin-walled diverticula called the foregut gland. The stomach contains a crystalline style in its median diverticu­lum. The stomach bears a pair of large folded midgut glands. The stomach continues as a coiled midgut (Fig. 16.3B) to end in the anus.

6. Locomotion of Neopilina:

There is a large flat ventral foot which occu­pies almost whole of the ventral side of the body. It is modified for creeping movement.

7. Respiratory System of Neopilina:

The respiratory organs of Neopilina are the gills attached to the dorsal side of the pallial groove. There are five pairs of gills in Neopilina galatheae while in Neopilina ewingi there are six pairs. The posterior gills have lamellae usually on one side, while the anterior ones have lamellae on both sides. The gills are supplied with blood by the afferent gill sinus to be returned by efferent gill sinus (Fig. 16.4).

8. Circulatory System of Neopilina:

The heart consists of two pairs of auricles and two ventricles. The ventricles are located on the lateral side of the hindgut and continue as a single aorta. The aorta supplies blood to the different parts of the body and is collected in different sinuses like other molluscs.

9. Excretory System of Neopilina:

The excretory organs are six pairs of nephridia. The nephridia are designated as the metanephridia which lie at the bottom of the pallial groove. A metanephridium has the appearance of a folded sac which opens to the exterior by nephridiopore near the gill. The metanephridia are modified coelomoducts.

10. Nervous System and Sense Organs of Neopilina:

The nervous system is primitively built and corresponds to the ladder-like arrange­ment of Chiton. Two ill-developed cerebral ganglia, one on each side of the pharynx are present. The cerebral ganglia are connected by cerebral commissure which encircles the mouth. The cerebral ganglion and its com­missure innervate the preoral tentacles, labrum, oral lobes and subradular organ.

The pedal cords innervate the foot. The pleural cords send nerves to the mantle, gills and nephropores. The sense organs of Neopilina include a pair of statocysts, a subradular organ and pre-oral tentacles. The pedal and pleural cords are connected posteriorly to form loops with many cross-connections between them. Such an arrangement in the nervous system speaks of primitive organisation.

11. Reproductive System of Neopilina:

Neopilina is a dioecious animal. The gonads are paired ventral organs (Fig. 16.4) which open into the third and fourth metanephridia. These metanephridia act as the gonoducts. The sperms and ova are discharged through nephropores. Accessory reproductive glands are lacking.

12. Relationships of Neopilina with other Molluscs:

Neopilina possesses many peculiar fea­tures which make the precise relationship of it debatable. The metameric arrangement of dorso-ventral muscles, gills, nephridia and gonads establish its phylogenetic relation­ship with the annelids. As regards its intraphylar position, it shows many similari­ties with Chiton, Nautilus and many gastro­pods. The Ideation of the body in the shell corresponds closely to that of Nautilus.

Fea­tures common in Neopilina and Chiton are the shape of body, the flat ventral foot, the shell covering the head and mantle many dorso-vental muscles, auricles, numerous gills, etc. But Neopilina differs from Chiton by having a single shell, statocysts and the nephropores functioning as the gonoducts.

It is claimed that the gastropods have evolved from Neopilina via Nautilus-like form which has a symmetrical shell. In gastropods, tor­sion of the visceral mass has resulted the asymmetrical configuration, reduction of dorso-ventral muscles into a single columellar muscle, reduction in the number of gills and metanephridia and many other features.