8 NILS ODHNER, NORTHERN AND ARCTIO INVERTEBRATES. III. OPISTHOBRANCHIA ETC. 



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

 parapodia very important. 



Parapodia reflexed över the back; shell whoUy or partly covered by the mantle 



Fam. Aplysiidce. 



C. Pleurobranohoidea (Notaspidea) : The dorsum forming a shield-like plate, notseum, 

 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 notseum, or wanting Fam. Pleurobranchidce. 



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



Fam. Umhraculidce. 



Subordo 11 Nudibranchia. 



No ctenidia; back generally with symmetrical processes or »papillee», 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 receptaculuni 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- 

 bulse present. 



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

 MAN 1890) ; body in transversal section quadrangular. 



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

 circular velum ; papillse ramified ; mandibulaR in the anterior margin of the 

 bulbus Avith processes; radnia broad, go . 1 . oo . 



+ . Vehmi with short digitiform processes or papillse; vaginse of the rhino- 

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



Fam. Tritoniidce. 



+ + . Velum smooth ; vaginse of the rhinophores with the margin bilobed ; 



the back with three series of processes Fam. Atthilidos. 



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

 teral lobes imited to the foot; papillse small, sparse-set on the back; man- 

 dibulse lateral without processes ; radula narrow, 1.1.1 



Fam. GoniceolididcB n. fam. 



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

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

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

 teral, Mdth processes; radula broad, oo . 1 . oo Fam. DendronotidcB. 



B. Doridoidea (Holohepatica) : 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- 

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



