of the SuhJiingdom Mollusca. 413 



circulatory system, therefore, would be of no use. The Platy- 

 lielmia seem to nie not to differ more from the Mollusca than 

 Peltogaster and Clistusaccus froui the Crustacea. In the ento- 

 zoic Crustacea some of the harder oral parts are generally pre- 

 served, in order to serve the animal for attachment. It is there- 

 fore likely that the hooks of Cestoda are homologous with simi- 

 lar hard organs of Mollusca. Gegenbaur* compares in this 

 way the four uncigerous retractile organs of Tetrarhynchus with 

 the cheek-hooks (harpagje) of the Pteropoda gymnosomata. 

 Perhaps a closer study of the genus Homoderma of Van Beneden f 

 (outwardly resembling a Distoma, but anatomically agreeing 

 with Pneumodermon) may throw some light on this question. 

 The hooks of the Cestoda may also be compared with the 

 lingual teeth of the Pellibranchiata, and chiefly with that part 

 called by Messrs. Alder and Hancock J the '^prehensile collar'" 

 in Limapontia nigra. These hooks probably assume a circular 

 arrangement. The lingual teeth are frequently absent among 

 Gasteropoda, e. g. Doridopsis (Hancock), Cirroteuthis and Tethys 

 (according to Bergh). 



A secretory organ (kidney) is never absent in any animal 

 taking food ; and it therefore exists in all Entozoa, agreeing in 

 form with the renal organ of Elysia as represented by Souleyet. 



The generative organs of the Platyhelmia agree exactly with 

 those of the androgynous jMollusca in general, but chiefly with 

 those of the genera § Chalidis and Pelta ; at least I cannot dis- 

 cover any notable ditference on comparing the anatomy of the 

 two latter genera with that of Amphistomum subclavatum as 

 represented by Dr. Walter ||. The Mirudines and Lumbrici, 

 which are considered by Prof. Quatrefages distinct classes, are 

 also androgynous, with reciprocal copulation; but the gene- 

 rative organs, like the nervous system, seem to be formed 

 according to quite another plan, although they may have some 

 distant resemblance to those of Elysia. It may be questioned if 

 androgyny is of sufficient importance to unite animals differing 

 so greatly in their exterior form, especially as hermaphroditism 

 is only of specific value among Fishes [Serranus scriba) and 

 Acephala {Pedeu operculai'is) . I, however, believe that andro- 



* Vergleichende Anatotnie, p. 329. 



t Exercices zoototniques, 1839, p. 54, t. 3. f. 4-6 (Mem. de I'Acad. 

 Roy. des Sciences de Bruxelles, tome xi.) 



X Ann. Nat. Hist. ser. 2. vol. i. p. 208, pi. 20. f. 2. Quite different from 

 " the prehensile collar " of the same authors in their ' British Nudibran- 

 chiata.' 



§ Quatrefages, Meraoire sur les Gasteropodes phlebenteres (Ann. Sc. 

 Nat. ."3 ser. 1844, tome i. p. 15.5, t. 4. f. 4) ; Morcli, " Contributions xx. la 

 Faune malacologique des Antilles danoiscs," I. c. p. 24. 



II Archiv fur Naturgcschiehte, xxiv. p. 2G9, t. 1 1 . f . 5. 



