22 PADL PELSENEER. 



be constructed upon the same plan, — that is to say, the genital 

 caeca are female in their terminal portion. 



(3) Entoconcha. — This genus, so degraded by parasitism, 

 has unfortunately not yet been studied by the method of serial 

 sections. As is known, there is a female genital gland in the 

 middle region of the body, and a number of spermatogenic 

 capsules further back towards the posterior orifice of the body. 



(4) Bulloidea. — The genital gland is here formed of acini, 

 all hermaphrodite (Bulla, Limacina, &c.) ; but iu certain 

 forms (Actseon, Pelta, Lobiger) I have found it composed 

 of distinct male and female acini. 



(5) Aplysioidea. — The same acini produce spermatozoa 

 and ova (Aplysia). I have found, however, that the male and 

 female acini are distinct in certain specialised forms ; the former 

 (male) occupy the central, and the latter the peripheral region 

 (Pneumonoderma, Clinopsis). 



(6) Pleurobranchoidea. — The genital gland is uniformly 

 composed of hermaphrodite acini in Umbrella. But in 

 Tylodina and all the Pleurobranchidse I find that the acini 

 are either male or female, and that the latter open into the 

 former. 



(7)Nudibranchi a. — Meckel was the first to notice in certain 

 Nudibranchs that the genital gland is formed of male and of 

 female acini ; but he supposed that these acini of different 

 sexes were without communication with one another, and that 

 in the genital ducts there was a male canal embedded in the 

 female one (5). 



Nordmann immediately afterwards showed that in Tergipes 

 the ovular acini open into capsules full of spermatozoa; but 

 he took the entire hemaphrodite gland for an ovary merely, and 

 held the sacs full of spermatozoa to be ''poches de fecon- 

 dation" (6). 



It was R. Leuckart who first determined and correctly inter- 

 preted the constitution of the hermaphrodite gland of Nudi- 

 branchs, especially Eolis (7), — i. e. peripheral acini exclusively 

 ovular, opening into central chambers producing spermatozoa. 

 His interpretation has been confirmed since for a great 



