GENERAL ZOOLOGY 



Class Mammalia. Terrestrial or secondarily aquatic forms, at least partially 

 covered with hair. Warm-blooded, lung-breathing, with mammary glands. Mostly 

 viviparous. Mammals. 



The Phylum Hemichordata 



The Balanoglossida: Saccoglossus. Species of this genus, and of such 

 other genera as Balanoglossus, Glossoba/anus, etc., are marine, worm-like 

 animals that usually Hve in shallow water and are found burrowing in muddy 

 or sandy bottoms. Externally, three body regions are apparent: the proboscis, 

 the collar, and the trunk (Fig. 18.1). The mouth is located ventrally, just 

 beneath the proboscis and within the anterior border of the collar; the anus 

 lies at the posterior tip of the trunk. The gill slits form a dorsolateral row 

 of perforations on each side in the anterior region of the trunk, extending 

 posteriorly for some distance. These perforations are passages from the cavity 

 of the pharynx to the exterior. Near the middle of the trunk region, transverse 

 ridges are produced by paired, glandular caeca which arise from the digestive 

 tract. The digestive system includes a buccal region within the collar, a 

 pharyngeal region marked by the gill slits, and an intestine which bears the 

 caeca and leads to the anus. As the animal burrows through sand or mud, 

 the proboscis is thrust forward, and the silt from which food is extracted 

 enters the mouth and pharynx. Excess water passes outward through the 

 dorsolateral gill slits, aerating the blood in vessels lining their walls. The 

 ingested silt, concentrated in a ciliated groove in the ventral region of the 

 pharynx, is conducted posteriorly through the intestine, where usable food 

 materials are digested and absorbed. 



Coelomic spaces are found within proboscis, collar, and trunk, being de- 

 rived from three pairs of primary embryonic coelomic pouches. The circu- 

 latory system consists of a median dorsal and a median ventral longitudinal 

 vessel, interconnected by small lateral branches in the pharyngeal region and 

 posteriorly. A so-called "heart" lies in the proboscis, but the blood is probably 

 propelled chiefly by peristaltic contractions of the longitudinal vessels. A 

 glomerulus, which has been considered an excretory organ, is connected with 

 blood vessels in the proboscis. 



The nervous system is composed of two principal nerve trunks formed as 

 ectodermal thickenings in the median dorsal and median ventral regions of 

 the trunk. These cords appear to represent specialized areas of a generally 

 distributed subepidermal nerve net, or plexus layer. The cords are connected 

 by a ring-like epidermal thickening in the collar, and the dorsal trunk pro- 

 ceeds anteriorly through the collar before dipping downward to enter the 

 proboscis. The resemblance of this nervous system to the central nervous 

 system of the typical chordate is not outstanding, although the nature of the 

 dorsal nerve trunk in the collar region is somewhat suggestive of the chordate 

 nerve cord. A structure called the stomochord is present in the posterior part 



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