GENERAL EEATl'RES, INIETHODS, ETC. 21 



attached near the raduhx. A definite digestive gland i.s wanting, tiie mid-gut 

 pouches being hned with hepatic cells. 



Owing to the great reduction of the foot and correlated changes, several 

 peculiarities appear in connection with the circulatory system. As in the 

 Chitons the heart is posterior, and the aorta passes along the mid line dorsal 

 to the gonad to connect with the head cavity, which in Limifossor is limited 

 posteriorly by a well-developed septum. In this genus there are indications also 

 of a pedal sinus, but behind the head region it largely disappears, the blood 

 flowing between the gut and body wall to the branchial region. Passing through 

 the ctenidia, or the folds in the cloacal wall, when these are present, the blood 

 makes its way to the posterior end of the heart. 



The nervous system bears a striking resemblance to that of the Chitons. 

 There is a greater concentration of the nerve cells to form well-differentiated 

 ganglia, but otherwise there are, in such species as Proneomenia liawaiiensis, no 

 especially unique features. The supraoesophageal mass originates three pairs 

 of nerves, which innervate the buccal and neighboring body walls and three 

 pairs of connectives, the labiobuccal, pedal, and lateral. The first named, in 

 a typical condition, is decidedly Chiton-like both as regards its position and 

 elements. The other two, passing backward throughout the entire length of 

 the animal, are united frequently by conunissures and connectives, and may 

 fuse completely (Chaetodermatidae) in the cloacal region. In a number of 

 other species the pedal cords, after diminishing in size in the hinder regions of 

 the body, may lack any connection with the lateral ganglia, or they' may termi- 

 nate in ganglionic enlargements (ganglion posterius iiiferius of Wiren) united 

 by connectives with similar swellings {gang. post, supcrius) on the end of the 

 lateral cords. The latter ganglia are invariably united by a heavy commissure 

 passing dorsal to the rectum, and the pedal cords likewise may be connected 

 by a subrectal commissure, thus completing a circiunrectal nerve ring. 



In the Solenogastres the secondary body cavity comprises that of the 

 gonad, pericardium, and the ducts leading from this latter space to the cloaca. 

 In the Neomeniina the species are hermai^hroditic; in the Chaetodermatina 

 dioecious. The sexual elements pass through the pericardial cavity into the 

 coelomoducts, which in an immature condition are relatively simple, and in 

 some species at least are not fused before they open into the cloacal cavity, 

 characters which the Chaetodermatina retain throughout life. In the Neo- 

 meniina, on the other hand, various modifications may occur which produce a 

 high degree of complexity. Two or more seminal receptacles are usually present, 



