288 INVERTEBRATE MORPHOLOGY. 



gland producing both ova and testes. This hermaphrodite 

 glaud is a hollow sac divided into two principal compart- 

 ments by a longitudinal partition and lies above the digestive 

 tract. It is a hollow structure (Fig. 128, ro), the reproductive 

 elements developing from the cells lining its walls and pass- 

 ing from its cavity into that of the pericardium (p), with 

 which the reproductive sacs communicate. They are in fact 

 simply prolongations of the pericardial body-cavity, and the 

 epithelium lining them is continuous with that of the pericar- 

 dium. From the pericardial cavity the ova and spermatozoa 

 pass to the exterior by the uephridia. 



The Solenogastres are especially interesting on account of the many 

 structural peculiarities of a primitive character which they present and in 

 consequence of which they have been regarded as representatives of ances- 

 tral Molluscan forms. By others, however, this important position is 

 denied them on the ground that many of their peculiarities are due to 

 degeneration produced in accordance with their life in the mud at the bot- 

 tom of the ocean. The absence of a shell, the reduction of the mantle- 

 lobes, foot, and radula may with plausibility be accounted for in this 

 manner, but there are other peculiarities that are certainly primitive 

 which are not thus explicable. The relation of the heart to the pericardium 

 is one of these, and others are the communication of the hermaphrodite 

 gland with the pericardium, and the functioning of the nephridia as ducts 

 for the reproductive organs. The Solenogastres are unquestionably primi- 

 tive Mollusca; the only question which is yet to be settled is to what extent, 

 if any, degeneration is responsible for their external peculiarities, such as 

 the absence of a shell, the reduction of the mantle-lobes and of the foot. 

 It must be noted in this connection that one form belonging to the genus 

 Dondersia has been described as passing through in its development a 

 stage in which indications of a shell consisting of several plates and simi- 

 lar to that of the Polyplacophora was present, a condition which would 

 seem to indicate the derivation of the members of this group from forms 

 provided with a distinct shell. 



2. Order Polyplacophora. 



The Polyplacophora, like the preceding order, contains 

 only marine forms. For the most part they are somewhat flat- 

 tened animals with a rather broad foot occupying the ventral 

 surface, while from the sides of the body a slight fold, the 

 mantle-fold, projects. In one genus, Ckitondlus, the form of 

 the body is more cylindrical and the foot is rather narrow 



