GENERAL ZOOLOGY 



Fig. 18.3. Subphylum Urochordata: simple ascidians. A, Ciona intestinalis, general external 

 appearance. B, internal anatomy of a sea squirt, Molgula. {A, photoa;raph by George Lower; 

 B, photograph of a model, courtesy American Museum of Natural History.) 



is firmly attached to the body only in the region of the siphons and can be 

 removed without disturbing the internal parts of the animal. The tunic 

 functions as a tough, elastic shell, although it contains the cells by which 

 it is secreted, as well as blood spaces through which nutrients reach these 

 cells. The principal constituent of the test is an organic compound which 

 has been named tunicin; this is very similar to cellulose, a compound com- 

 mon in plants but rarely produced by animal tissues. Removal of the test 

 exposes the true outer surface of the body. Most of this surface is the so- 

 called mantle, which enclo.ses an extensive cavity, the atrium. The excurrent 

 siphon is essentially a specialization of the mantle enclosing the median 

 portion of the atrial cavity, from which lateral portions extend on each side 

 beneath the mantle. Since the atrium is formed by the outgrowth of double 

 flaps from the outer surface, it is lined by epidermal cells. The mantle con- 

 sists of inner and outer epidermal layers, between which lie muscle fibers, 

 connective tissue, and blood sinuses. 



To understand the processes involved in feeding and in respiration, it is 

 necessary to understand the structural relationships between the digestive tract 

 and the atrium. The opening of the incurrent siphon may be called the 

 mouth, and the cavity within this siphon, the oral cavity. A circlet of tenta- 

 cles marks the beginning of the pharynx, or branchial sac, which is relatively 

 large and specialized for food collection and aeration of the blood. The re- 



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