286 INVERTEBRATE MORPHOLOGY. 



tract (r) and the nephridia («)' and contains the branchisa {ct). 

 These last are either a single pair of structures each consisting 



of a central axis with pinnately- 

 arranged lateral appendages or in 

 some cases are represented by 

 bunches of ciliated filaments. 



The ectoderm rests upon a 



layer of muscular tissue in which 



Fig. 137, —Diagram op Ar- both circular and longitudinal 



EANGBMBNT OF OuGANs AT layers cau be distinguished, and 



Hind End of Ghmtoderma jj^jjerous bands of transverse 



(after Hubrbcht from Lankester). 



a = ctemdium. ^^^^^' i^ some cases arranged to 



n = nepliiidium. form septa placed at regular 



= ovary. intervals, traverse the body- 



p = pericardium. ^^^._ ^ fairly-capacious peri- 



V = rGCtUID. . . 



cardium is present, lying dorsally 

 to the posterior portion of the digestive tract and into its 

 upper portion the heart projects slightly, not being, however, 

 completely enclosed by the pericardium. No auricles seem 

 to be developed, nor are any definite blood-vessels present, 

 the circulation being throughout lacunar. 



This condition of tile heart in relation to the pericardium is interesting 

 as showing its original independence of that portion of the body-cavity. 

 Its enclosure in the pericardium in the higher Mollusca is a secondary con- 

 dition, the heart and its cavity belonging to the schizocoelic structures 

 rather than to the so-called enterocoelic pericardium. This agrees perfectly 

 with the relationships of the blood vascular system of the Nemerteans and 

 Annelids. (See pp. 165 and 231.) 



The mouth is a longitudinal slit upon the ventral surface 

 of the body and opens into a pharynx provided usually with a 

 radula and with salivary glands, though both these structures 

 are absent in Neomenia. The intestine pursues a straight 

 course towards the anal opening, being, however, in some 

 forms pouched, owing to its constriction at more or less regu- 

 lar and close intervals by muscular transverse septa. The 

 walls of the pouches thus formed are glandular and represent 

 the digestive gland of other Mollusca, though in Chcefoderma 

 there is a simple outgrowth of the digestive tract which rep- 

 resents it more perfectly. 



