EXTEROPXEUSTA LIMITING MEMBRANES 579 



occur in a way similar to that seen in the lower Crustacea, where the contractions 

 of the intestines are able to set the body fluid in motion. 



The " heart vesicle " appears, according to recent researches, to be of ectodermal 

 origin, and thus cannot be considered as a ccelomic vesicle. 



VIII. The Limiting Membranes, the Proboseidal Skeleton, and 

 the Branchial Skeleton. 



Throughout the whole body of the Enteropneusta, the walls of 

 organs which are in contact are separated from one another by 

 structureless limiting membranes, which are to be regarded as 

 secretions of these walls. These limiting membranes must be thought 

 of as composed for the most part of two adhering laminae. The blood 

 vessels lie within the limiting membranes ; they represent a system 

 of spaces between the two laminae. 



In secreting the limiting membranes, the histological character of 

 the secreting walls is of no consequence. A muscle wall can secrete 

 a limiting membrane just as well as an epithelial wall. 



After the foregoing description the reader will be able to under- 

 stand without further assistance the occurrence and arrangement of 

 limiting membranes in the body. He will, for example, know that a 

 limiting membrane exists everywhere below the body epithelium, 

 secreted by that epithelium on the one hand, and by the parietal 

 wall of the coelom on the other. 



A similar limiting membrane must also occur between the visceral 

 wall of the coelomic sacs and the intestinal epithelium, as also between 

 the anterior and posterior walls composing the septa, which separate 

 collar from trunk, peripharyngeal cavities from collar ccelom, etc. etc. 

 At certain points, especially in the proboscis and on the branchial 

 intestine, the limiting membrane becomes thickened, and forms the 

 proboseidal and branchial skeletons. 



A. The proboseidal skeleton consists of a median body and two 

 limbs diverging backward. The body of the proboseidal skeleton lies 

 in the neck of the proboscis, between the neck of the proboseidal 

 diverticulum of the buccal cavity above, and the ventral body 

 epithelium of the neck of the proboscis below. The limbs diverge to 

 right and left into the collar region, clasping from above the entrance 

 to the buccal cavity, and in close contact with its epithelium. 



The proboseidal skeleton is further strengthened laterally by the 

 ehondroid tissue which becomes attached to it. The ground sub- 

 stance of this tissue is identical with the substance of the proboseidal 

 skeleton, and of the limiting membranes generally. It is secreted by 

 the anterior wall of the collar ccelom, and by the posterior wall of 

 the proboseidal ccelom, or by the latter alone ; but cell processes of 

 these walls remain in the secreted ground substance, and these processes 

 may break up into cell groups or nests, which give sections of the 



