THE SEA SQUIRTS OR ASCWIANS 



2945 



branching lines. The common apertures of discharge are distinct, and usually 

 furnished with lobes; the individual units are short and show no division of the 

 body into regions; and the outer tunic, which is usually soft, is traversed by 

 numerous vessels with large terminal knobs. Internal longitudinal bars are present 

 in the large and well-developed branchial chamber, in which the gill slits are 

 numerous; and the simple tentacles do not exceed sixteen in number. Budding 

 may take place either from the sides of the units constituting the colony or from 

 the vessels in the tunic. Among the five genera constituting the family, the 

 typical Botryllus (figured on p. 2949) has the colony thin and incrusting, with the 

 individuals arranged in a circular manner, whereas in 

 Botrylloides they form ellipses or branching lines. In the 

 figured species of the latter (B. albicans} from North- 

 western Europe and the Mediterranean, the colony is 

 pure white in color, but in some it is purple with yellow 

 or green markings, and in others yellowish green. As 

 an example of forms in which the colony is thick and 

 massive, we may cite the genus Polycyclus. Passing over 

 the second family of the suborder, we come to the third 

 {Polydinidce} , in which the colony is usually massive, 

 being sometimes incrusting, but in other cases lobed, or 

 even stalked. The arrangement of the individuals is 

 highly variable; acid the common apertures of discharge 

 are usually inconspicuous. Although of an elongated 

 form, the individuals usually differ from those of the 

 family last noticed by being divided into three regions; 

 the inhalent aperture having six or eight lobes, while the 

 exhalent is frequently provided with a tongue-like process. 

 The gelatinous or cartilaginous outer tunic is frequently 

 stiffened by imbedded grains of sand; and the branchial 

 chamber is usually small and poorly developed, with 

 minute gill slits and no internal longitudinal bars. The 

 tentacles are small and not numerous; the digestive tract AN 

 is extended posteriorly to a considerable distance beyond 

 the extremity of the branchial chamber; and budding takes 

 place from the end of the postabdominal region. The 

 family is represented by well nigh a score of genera, among which Amarutium may 

 be selected as an example, on account of its numerous species. Here the colony is 

 massive, being sometimes lobed or stalked; the mode of arrangement is usually com- 

 pound and irregular; and the individuals are elongated, with six lobes to the inhalent 

 orifice, and the postabdominal region elongated. The species figured on p. 2946 (A. 

 densum) is from Northwestern Europe, and is characterized by its grayish yellow 

 color, and the abundance of sand in the tunic; but other kinds may be black, orange, 

 or rosy red, or white. In our figure, a shows fully active individuals; while those 

 in the outer ring indicated by b assume a kind of torpid condition during the winter, 

 but give rise to fresh buds in the spring. 

 185 



INCRUSTING ASCIDIAN 



(Botrylloides) ON A SEA- 

 WEED. 



(Natural size.) 



