106 MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



a sinus around the stomach. Along most of the course of the efferent vessel blood ceca are 

 given off. and a large bunch also arises from the sinus. 



Nervous system. We have found that the nervous system of the Actmotrocha is more 

 highh developed in some species than in others and that it is subepidermal in character. In the 

 different species of the adult we also find that there is considerable difference in the degree of 

 development of the nervous system and that it is largely subepidermal. 



Caldwell (3) was the first to give a good description of the nervous system, although 

 Kowalevsky (11) recognized the existence of a lateral nerve and ganglion. Caldwell found the 

 cing nerve, a hollow nerve cord on the left side, and he speaks of two ciliated pits consisting of 

 nerve cells, ganglion, and nerve fibres. The description is so brief, that one can not say whether 

 or not the ganglion that he speaks of represents the ganglion that Kowalevsky (11) and Cori (4) 

 describe. 



Benham (2) finds no ganglion in P. austiraUs, but describes two small areas which, it seems 

 probable, are the same as Caldwell's " ciliated pits." He is the first to recognize the existence 

 of a lateral nerve on the right side as well as on the left, and he finds a nerve ring with a nerve 

 to each tentacle arising from it. 



Cori (4) describes a definite ganglion, a lateral nerve on the left side only and tentacular 

 nerves. He is the onlj- investigator who has published anything on the distribution of the ner- 

 vous tissue in the lophophoral organ. 



Andrews (1). Torrey (22), and Ikeda (9) have given very brief descriptions of the nervous 

 system, but the two former recognize the existence of a short lateral nerve on the right side as 

 well as a long one on the left side, while the latter speaks of a so-called brain ganglion and 

 nerve ring. 



The account which Andrews gives of the nervous system of P. wchiteeba is very brief, 

 since his paper deals only with the description of a new species. He only speaks of the lateral 

 nerve and makes no mention of a brain ganglion, ring nerve, tentacular nerves, or nerves to the 

 lophophoral organ. 



In general our observations on the lateral nerve of P. a/rchitecta agree with those of Andrews 

 and Torrey. The lateral nerve of the left side is quite conspicuous and extends from the anterior 

 end to a point about one-third from the posterior end of the animal. It runs along the lateral 

 body wall until it is almost in the region of the transverse septum, then it gradually passes 

 obliquely upward in close proximity to the left nephridial canal, and finally is seen embedded in 

 the ectoderm at the side of the anal papilla. From this point it passes around the base of the 

 anal papilla between the anus and the mouth, and then it begins to take the same course close to 

 the nephridial tube on the right side as it did on the left side, but it soon grows much smaller in 

 diameter and finally disappears (tigs. 78 to 67). A longitudinal section passing through the mouth 

 and aims shows the relation which the nerve cord bears to the ganglion and nerve ring (fig. 84). 

 Cori (4) figures such a section through P. psammophila, but he seems to have overlooked the 

 nerve cord or axis cylinder in this region. It is closely associated with the cells of the ganglion 

 and lies just a little, below the latter. In an oral direction from the ganglion is seen a section 

 through the nerve ring. 



If a cross section (fig. 85) is taken through the ganglion so as to cut longitudinally through 

 the nerve cord and if the section is stained deeply with iron hsematoxylin and eosin, it will show 

 plainly that there is no cavity in the cord, but that it is made up of a mass of fibres surrounded 

 by a nucleated sheath. Caldwell (3) considers the structure to he a hollow nerve cord; Benham 

 (2) says that it has semifluid contents and that he has been unable to make out any punctated 

 nerve substance; and Cori (4) states that it is an axis cylinder. 



We have endeavored to find some connection between the cord and the ganglion, but have not 

 been very successful. In the region of the ganglion i. e., between the mouth and the anus 

 the sheath of the nerve cord does not seem to differ in thickness or character from the same 

 structure in other parts. The cells of the ganglion, however, send out processes which in 

 sections are frequently seen applied to the sheath, but no connection between the fibres of the 

 nerve ring and those of the cord could be made out. 



