58 



LOWER VERTEBRATES. 



Fig. 49. —Colony of Botrtillus 

 composed of three systems. 



mantle, forming a massive gelatinous and frequently brightly-colored colony attached 

 to some stone, or frond of seaweed. In the different groups the individuals are 

 arranged in a characteristic manner, and the colony increases in size by different 

 modifications of the process of budding. In some the body is short and simple, in 

 others it becomes greatly elongated, and is frequently constricted so that it appears 

 to be composed of tvi^o or three segments, to which the names head, thorax, and 

 (third) abdomen are usually applied. 



In the BoTEYLLiD^, represented on our coast by JSotryllus gouldii, the body of the 

 individual zooids shows no sign of segmentation, and in the typical genus they are 

 arranged in a star-like manner around a common cloaca. The 

 general structure of one of the systems or groups of zooids is 

 diagrammatically represented in the accompanying cut, which 

 should be compared with the general figure of a tunicate at 

 the beginning of this article. The central cavity is the com- 

 mon cloaca, into which open the water-tubes coming from the 

 branchial sacs of the ^■ari()us individuals. The arrows show 

 the course of the currents of water. Hotryllus gouldii is very 

 common on the New England coasts, forming gelatinous 

 masses on eelgrass, algae, and hydoids, which 

 sometimes attain a width of an inch, and a 

 length of eight or ten. The color is very 

 variable. Our figure represents a small col- 

 ony of three systems. The larva was at first 

 considered as a composite, but more recent 

 researches show that this is not correct. The 

 larva, on settling down, forms an ascidian, 

 which by budding forms a colony. This 



budding process is very complicated, some three or four generations of buds being 

 foiTTied before a common cloaca is produced. The larva forms a bud and then dies. 

 This bud in turn develops two others and then disappears; the next generation con- 

 tains four individuals, and so on. These buds at first have no connection with each 

 other, but eventually they arrange themselves into groups, and take the structure 

 shown in the diagram. 



In the DiDEMXEDJE the viscera extend themselves behind the pharyngeal sac, and 

 the body is divided into two portions, dignified by the terms thorax and abdomen. In 

 the next group, the Poltclixid^, three divisions of the body — thorax, abdomen, and 

 post-abdomen — are present, and each individual is very long. Amarcecitim, a genus 

 common on our coasts, forms large colonies, some of which reach a foot or more in length. 

 The general color is much like that of boiled salt pork, although more translucent, 

 and the fishermen, who occasionally draw them up on their lines, call them sea-pork. 

 The animals are arranged much as in Hotryllus, each system having a common cloaca. 

 The process of budding has been studied in some of the species. The larva attaches 

 itself, and develops a solitary zooid, with its three body divisions. Soon the post- 

 abdomen separates from the rest, and divides into several sections, each of which 

 develops into an independent zooid, the parent in the meantime forming a new post- 

 abdomen. Each of the second generation repeats the process, and the result is that 

 the colony increases rapidly in all directions. 



Fig. 50 — Diagram of the structure of BotrijUus. 

 Tlxe arrows slmw the course of the water pass- 

 ing in through the mouths and out through 

 the central cloaca. 



