■.jH 



ZOOLOGY 



far the li ingest part of the body, while the pre-oral region almof3t completely 

 aliorts. \Vhen the post-oral region has attained a certain size, there are 

 developed on it t-rto lateral folds, the rndiments of the mantle (B), which grow 

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

 margins. The pre-oral circlet or velum changes its 

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

 and is then gradually reduced, the larva sinking 

 to the bottom ; and thoiigh 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 charac- 

 teristic three-lobed shape. 



^scU.at 



^ap 



B. RHODOPE. 



\\^ 



Rhodope (Fig. 652) is a minute, elongated, fusi- 

 form 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 

 calcareovis spicules. There are neither jaws nor odonto- 

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

 consisting of buccal cavity, with salivary glands, 

 oesophagus, mid-gut with a caecum, and rectiim — opens 

 in an anal aperture situated to the right of the 

 posterior extremity of the body. A digestive gland 

 is absent. The central part of the nervous system 

 consists of a supra-oesophageal mass made of three 

 pairs of ganglia — cerebro- visceral, pedal and buccal — 

 and a single ventral ganglion. An eye and a stato- 

 oyst are situated on each side in close relation to the 

 cerebro-visceral ganglion. The nephridial system 

 opens on the right side in front of the anus : it 

 consists of a narrow ciliated canal, running out from 

 which are two longitudinal excretory canals with a 

 number of flame-cells similar to those of the Flat- 

 worms, but of a multicellular structure. 



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 an- 

 terior being ovaries and the posterior testes. There 

 is a common duct receiving the products of all the 

 gonads ; and a single hermaphrodite aperture, with 

 a muscular penis, a receptaculum seminis, and an 

 accessory 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 doubt that 



Rhodope is best regarded as a degenerate member of the Mollusca, and it probably 



finds its nearest relatives among the Gastropoda. 



Firj. Gr.2. —Rhodope ver- 

 anii. General view. The 

 scattered curved bodies 

 are the spicules. ^ a^->. 

 male aperture ; ? Hji. 

 female aperture ; l)uc. 

 buccal cavity ; hrn, cen- 

 tral nervous system ; c(£c. 

 caecum ; int. intestine ; 

 iidh. mouth ; or. ovary ; 

 '[)<iiii. layer of pigment ; 

 sal . ,<//'/. salivary gland ; te, 

 testes. (After von Graff.) 



