i860.] On the genus Onckidium. 95 



animals. Direct observations as to the development of the embryos, 

 etc. remain, however, as yet a desideratum. 



Organs of respiration and circulation. (See fig. 5, pi. xiv). 



All the Onchidia are pulmoniferous, the respiratory cavity occupy- 

 ing about one-fourth of the posterior length of the body. This cavity 

 is situated dorsally immediately under the mantle, its internal walls 

 being folded and fitted out with a soft whitish largely cellular and 

 cavernous epithelium, the lungs ; it is anteriorly closed on the left 

 and open en the right side, and the former half is somewhat smaller 

 than the latter, The respiratory opening is a round hole, situated 

 on the lower side at, or near, the end of the mantle ; it is surrounded 

 by strong concentric muscles and has occasionally a swollen margin, 

 which can he expanded or contracted at will, sometimes also forming a 

 retractile tube. 



The cardial cavity lies on the right side about two-fifths dis- 

 tant from the posterior end, and in front of the respective larger half 

 of the lungs. It is very muscular and encloses the heart, which is 

 represented by a small, reddish, oval capsule, thicker posteriorly than 

 anterioily. The arterial blood enters the heart from behind in which 

 point, — save that they have lungs, — the Onchidid^: perfectly agree 

 with the Nudibranchiata of the Opisthobranchia, with which they 

 have so much common in the general form of the body. From the 

 heart issues in front only one thick artery, being at the beginning 

 attached to the wall of the mantle by numerous very thin muscles-. 

 A short distance from its issue, it divides in two branches, one supply- 

 ing the reproductive organs and the other the digestive system. The 

 latter branch again divides before entering that system, one portion 

 being reserved for the digestive organs, and the other supplying the 

 head ; this portion of the artery, accompanying the alimentary canal, 

 passes through the large ganglion. From all the internal organs, 

 numerous very thin threads issue, connecting them with the mantle 

 and the foot ; some of these threads are no doubt blood-vessels, and others- 

 of a muscular and nervous character. The venous blood appears to be 

 conducted to the lungs by an open capillary system, at least I did not 

 observe special vessels for that purpose. A very large number of capil- 

 lary tubes, connects the upper frontal portion of the pulmonary cavity 



