XII 



PHYLUM MOLLUSC A 



707 



far 

 aborts. 



the longest 



of the body, while the prai'-oral region almost completely 



<iuuiua. ..lien me post-oral region has attained a certain six.e, there are 

 developed on it two lateral folds, the rudiments of the mantle (B), which grow 

 inwards towards the middle ventral line, and later on unite by their free 



part 

 When the 



margins. The prae-oral circlet or velum changes its 

 form at first it is conical, later it becomes plate-like, 

 and then gradually becomes reduced, the larva sinking 

 to the bottom, and though still occasionally swim- 

 ming with the aid of the velum, coming to use the 

 foot as a creeping organ. The shell now increases 

 in size step by step with the growth of the mantle, 

 .and bends round the body of the larva until its edges 

 meet and coalesce in the ventral median line. Later 

 it assumes the elongated conical form, curved towards 

 the dorsal side, characteristic of the adult. The foot 

 at the same time elongates and takes on the character- 

 istic three-lobed shape. 



mtfi 



sajffl 



int 



. 



B. RHODOPE. 



Rhodope (Fig. (514, bis) is a minute, elongated, 

 fusiform animal, ciliated externally with complete 

 .(external) bilateral symmetry. There is no shell, but 

 within the body-wall, in the parenchyma between it 

 and the enteric canal, are numerous irregularly shaped, 

 -calcareous spicules. There are no jaws or oilonto- 

 phore. The enteric canal, which is a narrow tube, 

 consisting of buccal cavity, with salivary glands, 

 esophagus, mid-gut, with a cnecum, and rectum, 

 opens in an anal aperture situated to the right of the 

 posterior extremity of the body. A liver is absent. 

 The central part of the nervous system consists of 

 two ganglia situated close together above the oeso- 

 phagus, and a single ganglion below. A pair of lateral 

 nerve cords run backwards from the posterior of the 

 two upper pairs of ganglia. There are a pair of eyes 

 and a pair of otocysts situated close to the posterior 

 upper ganglia. 



The nephridial system consists of a chamber open- 

 ing on the right side in front of the aims ; into this 

 open nine or ten flask, -shaped flame-cells similar to 

 those of the Flat-worms. 



There are no blood-vessels, and specialised organs 

 of respiration are also absent. 



The sexes are united. The gonads consist of about 

 twenty ventrally situated masses of cells, the aiiterioi 

 being ovaries and the posterior testes. There is a 

 common duct receiving the products of all the gonads : 

 anteriorly this divides into spermiduct and oviduct 

 with separate apertures situated on the right side, the 

 spermiduct with a muscular penis, the oviduct with a 

 receptaculum seminis and an accessoi'y gland. 



There is no metamorphosis, and the larva is not provided at any stage with 

 any representatives of either shell-gland or foot. 



" Though the occurrence of flame-cells is unique there can be little dcmht that 

 Rhodope is best regarded as a degenerate member of the Mollusc i. and 

 probably finds its nearest relatives among the Gastropoda, 



to. 014, b!#. Rhodope 

 veranii. (icneral view. 

 The scattered curved 

 bodies are the spicules. 

 cJ ap. male aperture; 

 9 I, iji. female aperture; 

 !,!<. liuccal cavity ; '</v. 

 central nervous system ; 

 ...... cifcum ; int. intes- 

 tine ; nitii. month ; ov. 

 ovary ; .//. <il<l. salivary 

 gland: tt. testes. (After 

 von Graff.) 



