58 



LOWER VEIITEURATES. 



Fig. 49. —Colony of Bntriillus 

 composed uf tliree 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 ajipears 

 to be comi)osed of two or three segments, to whicli the names head, thorax, and 

 (third) abdomen are usually applied. 



In the BoTRYLLiD^, represented on our coast by Botrijllus (/oiildii, 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 accora]ianying 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 various individuals. The arrows show 

 the course of the ctirrents of water, liotrijllus gouldil is very 

 common on the New England coasts, forming gelatinous 

 masses on eelgrass, algae, and hydoids, which 

 sometimes attain a width of an indi, and a 

 length of eight or ten. The color is vei-y 

 variable. Our figure re])resents 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. Tlie 

 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 

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

 This bud in turn develops two others and then disap]iears; 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 groujis, and take the structure 

 shown in tlie diagram. 



In the DiDEMNin.E the viscera extend themselves behind the jiharyngeal sac, and 

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

 the next group, the Poi.TCLiNiDyE, three divisions of the body — thorax, abdomen, and 

 post-.abdomen — are present, and each individual is very long. Ainaroecitmi, 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 tlieir lines, call them sea-]iork. 

 The animals are arranged much as in JBotrijllns, 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 jiost- 

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

 develops into an independent zooi<l, the parent in the meantime forming a new |)o.st- 

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

 the colony increases rapidly in .all dii'ections. 



. — IMagraiii of llie structure of Bofrtjlhts. 

 s arrows sIimw tile course of the water pjiss- 

 : in tlirou^Ii the ntouLhs and out through 

 central cloaca. 



