36 COMPARATIVE ANATOMY. 



the inner face of each Uxbio-buceal gangUon and unites with a small subradular 

 ganglion which is in close contact with a subradular organ. These two ganglia 

 are in turn united by a subradular commissure. In the genus Chaetoderma 

 also there is a well-defined subradular system (p. 57). 



In several species of Solenogastres various authors have foinid what corre- 

 s]5onds to tiic labio-buccal connectives and ganglia, and in most of these cases 

 have found one connnissure which is either the ventral buccal or labial. In 

 the Neomeniidae the subradular organ is usually wanting together with the 

 customary nerve sup]ily; and in most species it is impossible to find more than 

 one buccal commissure. However these nerves are usually very small and 

 difficult to trace so that negative evidence in this case may not be entirely trust- 

 worthy. 



Sense organs. — In the majority of Solenogasti-es a dorsal sense organ 

 exists in the mid line in the cloacal region. In the Neomeniina it consists of a 

 circular depression, naked or covered with a thin cuticular layer, and surrounded 

 by spicules which in its contracted condition overarch it. When expanded by an 

 underlying blood sinus a disc-like projection is elevated from the bottom of the 

 depression and is raised above the surrounding spines. In the Chaetodermatina 

 a groove, likewise overarched by spi.cules when contracted, is probably a homol- 

 ogous structure. In both families these organs are innervated by nerves from 

 th(^ dorsal commissure uniting the lateral nerve cords. Various functions have 

 been assigned, but without any experimental evitlence. 



Thiele believes that the ventral fiu-row may be tactile but no sense cells 

 have been shown to exist. In the gills on the other hand stiff hairs have been 

 found among the cilia and are considered to be parts of sensory elements. 

 Osphradia are unknown. 



In the Chaetodermatitlae the anterior sensory i)late is innervated by a 

 heavy set of nerves and probably acts as a tactile organ. Owing to the heavy 

 cuticular covering it may act also as a digging organ, operating in a general way 

 like a hog's snout. 



In Proneinuoiid lumHuiinisis a low sensory ridge encircles the atrial wall 

 innnediately within its outer opening. It is coni])osed of high colunuiar cells 

 which rest throughout their entire extent upon a rod-like mass of nerve cells. 

 Internal to this are the usual atrial ridges (Mundleisten), of which the more 

 external closely parallels the sensory tract just mentioned, while the inner one 

 passes nearly around the canal at the commencement of what is probably- the 

 mouth cavity. Both are usually well-developed ciliated folds, capable, in some 



