282 COMPARATIVE ANATOMY CHAP. 



follicles are said to unite to form a common duct. If this is really the case, then 

 the gonadial follicles together form a hermaphrodite gland. The hermaphrodite 

 duct, which runs forward, is said to divide into an oviduct and a vas deferens. 

 The latter leads to the muscular penis, which can be protruded from the male genital 

 aperture on the right anteriorly. With the oviduct are connected a receptacu- 

 lum seminis and a gland (albuminous or nidamental gland). The female genital 

 aperture is said to lie on the right side, behind, and distinct from, the male 

 aperture. 



A differentiated blood vascular system has not been found. A well-developed 

 body cavity is, however, present, filled with colourless nutritive fluid, in which blood 

 corpuscles are suspended. 



Special respiratory organs are wanting. 



The nephridial system has been described as follows. To the right, in front of 

 the anus, between the latter and the genital aperture, lies the outer nephridial 

 aperture. It leads through a short ciliated canal into a spacious renal chamber, 

 which is a widening of a longitudinal canal. The renal chamber bulges out at 

 several points to form short caeca. Into this chamber nine or ten small flask-like 

 organs open ; these resemble the excretory ciliated cells of the Platodes, inasmuch 

 as "flames" 1 arise at the base of each flask, the neck of which opens into the 

 chamber. 



Development is direct. At no stage are there any indications of a shell gland, a 

 shell, or a foot. 



Systematic Position. Rhodope is by some classified among the Turbellaria 

 (near the Rhabdoccelidce), by others among the Mollusca (near the Nudibranchia), 

 while others again are inclined to see in it a transition form between these two 

 phyla. 



There is apparently only one single point to support the theory of the relation of 

 Rhodope to the Turbellaria, viz. the presence of the ciliated excretory cells in the 

 nephridial system. On the other hand, the derivation of the nephridial system of 

 Rhodope, with its renal chamber and aperture to the right, from that of the Nudi- 

 branchia appears far more probable than its derivation from the water vascular 

 system of the Platodes. The presence of a rectum and anus, and of an infra-ceso- 

 phageal ganglion (pedal ganglion), is difficult to reconcile with a relationship to the 

 Turbellaria. The occurrence of an infra-oesophageal commissure in one isolated case, 

 that of Microstoma lineare (cf. vol. i. p. 166), is hardly a convincing argument. The 

 genital apparatus of Rhodope is much nearer to the Nudibranchiate than to the 

 Turbellarian type. 



There are, no doubt, serious obstacles in the way of those who seek to establish 

 the relationship of these animals with the Mollusca. The chief of these is the want 

 of a heart and the entire absence of a shell and a foot, even in the embryo. The 

 question to be decided is whether it would be possible for a Mollusc which had lost 

 foot, gills, and shell (e.g. Phyllirhoe) by the further loss of the heart, so far to depart 

 from the typical organisation of the Mollusca, that these organs would not appear, 

 even temporarily, in the course of development. If this question is answered in the 

 affirmative, then the asymmetry of Rhodope, and especially the position of the 

 genital, nephridial, and anal apertures on the right side, which entirely agrees 

 with their position in the Nudibranchia, affords strong support to its claim to be 

 related with the Mollusca. 



The view that Rhodope is a transition form between the Turbellaria and the 

 Mollusca need hardly be treated seriously. 



1 Cf. vol. i. p. 152, where flame cells are described. 



