302 W. J. CROZIER 



by a sphincter (Bordas, '99). Posteriorly the cloaca and body 

 wall are continuous in the region of the anal sphincter. The 

 longitudinal muscles of the body wall are reduced to very fine 

 threads just before they reach the anal sphincter, but there are, 

 in addition to the circular fibers of the sphincter, certain con- 

 tractile elements set perpendicularly to them. This last men- 

 tioned set of muscles assists in the opening of the sphincter; its 

 members are homologous to the radiating muscles of the cloaca. 



Concerning the innervation of these structures it is difficult to 

 get precise information, since the nervous elements are refractory 

 to ordinary neurological technique. I was unable to secure 

 methylene-blue preparations in the case of Stichopus. The 

 cloaca is referred to (e.g., Haanen, '14, p. 223) as of ectodermal 

 origin, in agreement with which is the fact that the inner cloacal 

 surface shows, though to a lesser degree, the pigmentation of 

 the outside of the animal. It is, therefore, possible that the 

 cloaca (including the radiating muscles [?] and sphincter) is 

 innervated from the radial nerve strands (Ludwig, '89-92, p. 

 70), in common with the body-wall muscles. The gut of holo- 

 thurians is supplied with special nerve strands from the nerve 

 ring, and the cloaca may receive a nervous supply from this 

 source. The holothurians have no aboral nerve ring. 



With reference to the finer histological condition of the nervous 

 apparatus, Bethe ('03, p. 22) remarks that "iiber die histologischen 

 Verhaltnisse des Nervensystems ist bei den Echinodermen so 

 gut wie Nichts bekannt," but it seems certain that a subepithelial 

 plexus is present in the form of a true nerve-net (Jordan, '14, p. 

 380). This would appear to be the condition in many autono- 

 mous organs of invertebrates, and especially in structures which 

 exhibit rhythmic contraction (Alexandrowicz, '13). 



b) The sequence of movements. Peristaltic movements appear 

 in the intestine, respiratory trees (as noted by Henri, '03), cloaca, 

 anal sphincter and body wall of Stichopus. All but the first of 

 these organs are concerned with the transport of water. In 

 ordinary inspiration the movements appeared in the following 

 order: 1) the anal sphincter opened, 2) a wave of opening moved 

 anteriorly along the cloaca, 3) the stalk of the respiratory trees 



