5 6S 



SEMI I ER TEBRA TES. 



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

 typical Botryllus (figured on p. 572) 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 colour, 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 

 {Poly din Ida), 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 ; and 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 embedded 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 

 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 AmaruciuTn 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 arrange- 

 ment is usually compound and irregular ; 

 and the individuals are elongated, with 

 six lobes to the inhalent orifice, and the 

 postabdominal region elongated. The 

 species here figured (^4. densum) is from 

 North- Western Europe, and is character- 

 ised by its greyish yellow colour, and 

 the abundance of sand in the tunic; but 

 other kinds may be black, orange, or 

 rosy red, or white. In our figure, " 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. 



Omitting mention of the remaining families of the group just considered, we 

 come to the third and last suborder of the typical ascidians, namely, the phos- 



AN INCRUSTING ASCIDIAN 



{Botrylloides) on a sea- 

 weed (nat. size). 



A COMPOUND ASCIDIAN (A iiinri'ri )i,n) IN WINTER 



condition (nat. size). 



