330 



HISTOLOGY 



to throw off such impurities as it can in contact with the air (dissolved 

 in water). No covering could be detected between the blood stream 

 and the surrounding epithelial cells in which the vessel lay embedded. 

 This form of tissue is much like that of 

 the lobster. 



The mollusks that breathe water have 

 a varied assortment of evaginated fila- 

 ments, plates, etc., which are in some 

 cases exceedingly complicated. We shall 

 study the conditions first as shown in the 

 gill of the prosobranch gasteropod, Syco- 

 typus (Fig. 294). This gill is a double 

 series of plate- or leaf-like evaginations, 

 between whose double walls the blood 

 slowly flows in very irregular capillary- 

 like sinuses. These vessels are separated 

 from the proximal edge of the epithelium 

 by an abundance of loose connective tis- 

 sue. 



The epithelium is columnar and dense, 

 and is as unspecialized a form of respir- 

 atory membrane as we have encoun- 

 tered. It is ciliated in many places and 

 contains many mucous cells. The base- 

 ment membrane on which the cells lie is well defined, but very thin in 

 most places. On the two sides of the edge of each lamella it is thickened 

 into two rods which are almost crescent-shaped in section. These are to 

 be considered as skeletal structures used to stiffen the gill plate (Fig. 



2 95)- 



The essential histological points of this respiratory tissue may be seen, 

 accompanied by far more complicated anatomy, in the ctenidia of other 

 mollusks. The squid and various lamellibranchs afford structures 

 that will repay study by the exhibition of marvelous adaptations. Most 

 of these other forms show a more specialized epithelium ; one whose cells 

 have given up other functions, and become as thin in body and as clear 

 in cytoplasm as possible to permit of the ready passage of gases through 

 their bodies. 



The respiratory tissues of the fishes are found on evaginations and 

 growths from the branchial arches. These take the form of a series of 

 lamellae which have arisen from the epithelium and its supporting con- 

 nective tissue. 



In the gill of the goldfish the general surface of the gill plate is cov- 

 ered by a stratified epithelium. The basal layer of this epithelium is 



FIG. 294. Central part and epithelium 

 on one side of a gill plate of Sycotypus. 

 bl.sp., blood space containing a thin 

 coagulum and a few blood cells; 

 res.ep., respiratory epithelium, x 



