THE CCELOM. 407 



cephalic appendages. Subsequently the larva rotates, bringing its neural side 

 up (cirripeds, tunicates, echinoderms, polyzoa, brachiopods (?)), and the 

 haemal surface of the head rapidly grows into the characteristic voluminous stalk, 

 or peduncle, by which the animal is permanently attached. 



The Ca'lom. The primitive body cavity is the space enclosed between the 

 somatic and splanchnic mesoderm. It was probably primarily segmented, form- 

 ing two completely closed chambers for each metamere. On the sides and haemal 

 surface, the walls separating adjacent chambers tend to break down, forming 

 extensive sinuses containing loosely united or isolated cells (blood cells). On the 

 neural surface the original segmentation is usually more strongly marked and 

 more permanent, and certain portions may be retained, or set apart, as thin 

 walled chambers, lined with epithelium, and devoid of free amoeboid cells or 

 blood corpuscles. 



They may consist of a small part of a single mesoblast segment, the part 

 that is directly connected with the excretory or nephridial duct, or that developed 

 mainly as a tubular outgrowth from it (Figs. 279, 294); or several such portions 

 may unite to form more extensive chambers. They are lined in part by flat, 

 indifferent endothelium, and in part by more specialized excretory cells, and 

 they may open to the exterior by glandular ducts, or nephridia, of which only 

 the terminal portion is of ectodermic origin. These so-called ccelomic chambers 

 are often referred to as the true ccelom, and have been regarded as the primitive 

 body cavity, but they are in reality either special portions of more primitive and 

 more extensive spaces, or the parts that remain hollow, after the other portions 

 have been shut off as vascular spaces or canals, or have been completely filled by 

 the growth of fibrous, muscular, or other tissues. 



