286 



IN VERTEBRA TE MORPHOL OOT. 



FIG. 127. DIAGRAM OF AR- 

 RANGEMENT OF ORGANS AT 

 HIND END OF Chceloderma 

 (after HUBRECHT from LANKESTER). 



ct = ctenidium. 

 n = nephridium. 

 o = ovary. 

 p pericardium. 

 ? = rectum. 



tract (r) and the nephridia (?i) and contains the branchiae (ct). 



These last are either a single pair of structures each consisting 



of a central axis with piunately- 

 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 

 both circular and longitudinal 

 layers can be distinguished, and 



^ CJ 



numerous bauds of transverse 

 fibres, in some cases arranged to 

 form septa placed at regular 

 intervals, traverse the body- 

 cavity. A fairly-capacious peri- 

 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 the 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 schizoccelic 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 usuall} T 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 Chcetoifcnmi 

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

 resents it more perfectly. 



