COMPARATIVE ANATOMY. 33 



longitudinal Innidles. In the anterior and posterior regions these are subject 

 to various modifications, forming mouth and 'cloacal sphincters and dilators, 

 and affording attachment for the gill retractors or anteriorly for the multitudinous 

 muscles operating the forward section of the digestive tract. These last named 

 muscles have never been fully studied in any species though they are fairly well 

 known in Chacloderma nUidulum and Limijossor talpoideus. As the matter 

 rests at present there is a similarity between the various species with respect 

 to the somatic musculatiu'e and to certain of the more conspicuous bundles 

 elsewhere in the body, but beyond this our data are too insufficient to permit 

 . of close comparisons. 



Primary Body Cavity and Septa. — The sjiace between the alimentary 

 canal, gonad, and body wall, the primary body cavity, reaches a varying height 

 of development according to the species. In the Chaetodermatina it constitutes 

 a comparatively limited pseudovascular system and in some of the Neomeniina 

 it is likewise much reduced, but in several species it becomes much more 

 extensive. This haemocele is divided by a horizontal septum that extends 

 between the longitudinal fibres on each side of the body beneath the gut and 

 so forms a ventral blood sinus. In the Neomeniina it is small, but is bounded, 

 in every species described in the present paper, by connective-tissue fibres per- 

 forated here and there to permit of communication with the overlying blood 

 spaces. In Chaetoderma a vertical septum separates heart, gills, and pallial 

 complex from the remainder of the primary body cavity. It is not present in 

 the Neomeniidae nor in Limifossor. In the last named genus there is an addi- 

 tional partition, essentially the same as in the Chitons, which separates the 

 head cavity from the succeeding portions of the haemocele. It is perforated 

 by the aorta, alimentary canal, and a pedal sinus which passes forward into 

 close proximity to the mouth. 



Circulatory and Respiratory Systems. — The heart is developed as a 

 fold of the dorsal pericardial wall reinforced by a varying number of muscle 

 fibres, and in some species differentiated into an auricle and ventricle. From 

 its anterior end the aorta arises and passing dorsal to the gonad leads into a 

 more or less definite head cavity. In the Chaetodermatina and a very few 

 Neomeniina this vessel possesses definite walls; in the others it is part of the 

 general lacunar network. In Limifossor a distinct connective-tissue septum 

 bounds the head cavity posteriorly, but in the greater number of other species 

 the blood spaces in the head region communicate directly with those surrounding 

 the mid gut. These last named sinuses are in communication also with a ventral 



