22 



ZOOLOGY 



SECT. 



times almost gelatinous, transparent or translucent, sometimes 

 tough and leathery, occasionally hardened by encrusting sand- 

 grains or fragments of shells, or by spicules of carbonate of lime. 

 The apertures always have the same position and relations, varying 

 only in their relative prominence. The pharynx varies in its size 

 as compared with the rest of the internal parts, in the position 

 which it occupies with regard to the various parts of the alimen- 

 tary canal, and in the number and arrangement of the stigmata. 

 The tentacles are sometimes simple, sometimes compound ; and 

 the dorsal lamina may or may not be divided up into a system of 

 lobes or languets (Fig. 682, lany.). 



In the composite Ascidians, as mentioned in the summary, the 

 zooids are embedded in a common gelatinous mass. The gela- 

 tinous colony thus formed is 



t/ 



sometimes flat and encrusting, 

 sometimes branched or lobed, 

 sometimes elevated on a longer 

 or shorter stalk. In certain forms 

 (Psammapilidium) the gelatinous 

 substance is hardened by the in- 

 clusion in it of numerous sand- 

 grains. The arrangement of the 

 zooids presents great differences. 

 Sometimes they occur irregularly 

 dotted over the entire surface 

 without exhibiting any definite 

 arrangement ; sometimes they 

 are arranged in rows or regular 

 groups; in Botryllus (Fig. 681) 

 they are arranged in star-shaped, 

 radiating sets around a common 

 cloacal chamber into which the 

 at rial apertures of the zooids 

 lead, while the oral apertures are 

 towards their outer ends. In 

 essential structure the zooids of 

 such colonies (Fig. 682) resemble the simple Ascidians. 



In the free-swimming pelagic Doliolum (Fig. 683) the shape is 

 widely different from that of the ordinary fixed forms. The body 

 is cask-shaped, surrounded as by hoops by a series of annular 

 bands of muscular fibres (mus. Ms.). The oral and atrial apertures 

 (or. ap.,atr. ap.) instead of being situated near together at the same 

 end of the body, are placed at opposite extremities, and the 

 relations of the various organs have undergone a corresponding 

 modification. The test is thin and transparent. Surrounding 

 each opening is a series of lobes the oral and atrial lobes in 

 which there are sense-organs ; and the first and last of the 



FIG. 681. Botryllus violaceus. or. 



oral apertures ; cl. opening of common 

 cloacal chamber. (After Milne-Edwards.) 



