90 On the genus Onchidium. [No. 2, 



the number and size of the pustules on the mantle, nor to the relative 

 narrowness, or width, of the foot. This is a very important statement 

 as regards the classification of the animals, and I shall endeavour to 

 prove its correctness by some observations which I shall subsequently 

 put upon record. 



The head is posteriorly on either side connected with the foot by 

 a thin membrane. 



Anatomy of Onchidium typhce. 



The respective places which the digestive and generative organs 

 occupy divide, so to say, the entire cavity of the body into two parts. 

 Figure 2 on plate xiv represents a specimen, opened along the entire 

 length of the centre of the mantle, the portions of which are removed 

 a little on the sides. The albuminous string of the penis is also a 

 little lifted up, and placed from the right to the left side, so as to allow 

 the ganglion and the penis to become visible. All the other internal 

 organs are in their original position ; the head with the oesophagus (oe), 

 salivary glands, (sg) ; alimentary canal (ac), &c. The signification of 

 the principal other letters is as follows ; pe. = pedicle ;p = penis with 

 the vas deferens twisted round it ; and (ps) the supplementary albumin- 

 ous string ; ng = principal nervous ganglion ; the digestive organs with 

 the liver (1) and the anterior portion of the stomach (st), rectum (r), 

 &c, are visible ; the generative organs with the ovarium (o), testis (t), 

 large albuminous gland (ag), receptaculum seminis (rs), &c. ; ht = 

 heart; I — lungs ; g and v = the hermaphrodite genital opening, a = 

 anus ; ol = pulmonary orifice. The digestive organs, thus roughly 

 estimated, occupy the greater portion of the front part, and the 

 generative organs that of the hinder part of the body. 



In order to understand more clearly the anatomical details, I must 

 direct the attention to figure 5 of plate xiv. This figure represents a 

 very large specimen of Onchidium typhce ; the foot has been along its 

 anterior and posterior, and the entire left basal margin detached from 

 the body and folded over to the left side, then the mantle has been . 

 cut in two halves and the left half (d) also removed laterally, so as 

 to join the other half only at the pulmonary orifice. The digestive 

 organs have been exposed in the figure on the right and the generative 

 organs on the left side. 



