8 



NILS 0DHNER. NORTHERN AND ARCTIC INVERTEBRATES. III. OPISTHO P.RANCHIA ETC. 



B. Aplysoidea (Anaspidea): No cephalic disc; tentacles and rhinophores well developed ; 

 parapodia very important. 



Parapodia reflexed over the back; shell wholly or partly covered by the mantle 



Fam. Aplysiidce. 



C. Pleurobranchoidea (Notaspidea) : The dorsum forming a shield-like plate, nota?um, 

 on or in which a shell is usually placed ; no head-shield nor parapodia. Cteni- 

 dium inserted in the middle of the right side and stretched backwards. 



1. Shell in the nota?um, or wanting Fam. Pleurobranchidce. 



2. Shell external, patella-shaped, with a nearly central apex 



Fam. Umbraculidce. 



Subordo II Nudibranchia. 



No ctenidia; back generally with symmetrical processes or »papillär, mostly 

 functioning as gills ; no mantle-cavity ; shell present only in earlier stages of evolution. 



A. Tritonoidea: Heart on the right side of the median line; male and female ductus 

 genitalis with joint orifices; vagina with receptaculum seminis only; liver com- 

 pact or divided in one posterior and two anterior parts with or without diver - 

 ticles to the dorsal processes: no blood-gland near the central ganglia; mandi- 

 bula? present. 



1. Papillae containing no liver-processes and without respiratory function (Herd- 

 man 1890) ; body in transversal section quadrangular. 



a. Rhinophores retractile within vagina? ; no oral tentacles; head with a semi- 

 circular velum; papilla? ramified; mandibular in the anterior margin of the 

 bulbus with processes; radula broad, oo . 1 . oo . 



+ . Velum with short digitiform processes or papilla?; vagina? of the rhino- 

 phores everted at the top ; the back with lateral appendages only 



Fam. Tritoniidm. 



+ +. Velum smooth; vagina? of the rhinophores with the margin bilobed; 

 the back with three series of processes Fam. Atthilidoe. 



b. Rhinophores without vagina?; oral tentacles small; head with produced la- 

 teral lobes united to the foot; papilla? small, sparse-set on the back; man- 

 dibular lateral without processes; radula narrow, 1.1.1 



Fam. Goniceolididce n. fam. 



2. Papilla? ramified without liver-processes and respiratory function (Herdman 

 1890); body slug-shaped; the vagina? of the rhinophores with ramose marginal 

 lobes ; no oral tentacles ; head-margin with branched processes ; mandibula? la- 

 teral, with processes; radula broad, oo . 1 . oo Fam. Dendronotidoe. 



B. Doridoidea (Holohepaticaj : Heart in the median line; female ductus genitalis di- 

 vided into two parts, each sometimes with an orifice of its own united with or 

 separated from the male one ; vagina with a spermatocyst in addition to the re- 

 ceptaculum seminis ; liver undivided ; a blood-gland near the central ganglia ; man- 

 dibula? usually absent ; gills surrounding the dorsal anus (except in Doridoxa). 



