22 



THE NAUTILI'S. 



by blood which has just left the alimentary tract and later 

 received its supply of oxygen. 



Brood Pouch. Fig. 8 represents a fully formed brood pouch 

 as seen in frontal section. The pouch has two distinct walls, an 

 outer and an inner. These are direct continuations of the 

 heavier portions of adjacent gill filaments. The outer wall con- 

 sists of a thin one-celled membrane made of flat expanded cells. 

 This wall is, in every respect, similar to the membranous part 

 of the gill filaments. The inner wall is also made up of a single 

 layer of cells. A part of this wall is similar to the outer wall 

 though the major portion is composed of very thick glandular 

 cells. Between the outer and the inner walls is a blood space. 

 This space is a modification of the spaces of the two filaments 

 to which the two walls are attached (Fig. 11). Numerous web- 

 like cross threads occur in the blood space. These are similar 

 to those which are found in the spaces of typical filaments. 

 They furnish another proof that the two walls are mere modifi- 

 cations of filaments. The brood pouch may contain but one 

 embryo (as in Fig. 8) or it may enclose a number (as in Fig. 

 11). A pouch may involve two filaments and only two (as in 

 Fig. 8) or it may be constructed from the parts of several. 

 The inner wall of such a pouch is thrown into folds which di- 

 vide it into communicating chambers. These folds probably 

 represent the contributions of the several filaments. 



Just how the pouch originates is still an open question. 

 StepanofT (1865) and Schereschewsky (1913) believe it to be a 

 modification of gill filaments. In Calyculina and Sphaerium 

 all the available evidence points to such an origin. The wax 

 model shows the pouch to be a modification of ordinary fila- 

 ments. The same filaments enter into the structure of the 

 pouch throughout its extent. 



PoyarkorT (1910) offers this theory for the origin of the pouch: 

 •* When the embryo comes into contact with the gill filaments, 

 it is surrounded and enclosed by leucocytes. Later these ar- 

 range themselves in two layers forming the brood pouch." He 

 considers *'the incubation of embryos in Cyclas as a case of 

 ectoparasitism accompanied by the formation of a follicle at 

 least in part, perhaps altogether mesodermal." Schereschewsky 



