24 



ZOOLOGY 



SECT. 



ties n e 



oto 



or.cip 



animal is enclosed : undulatory movements of the tail cause a 

 current of water to flow in through a pair of incurrent apertures 

 and out through a single excurrent aperture ; the former are closed 

 by lattice-work of fine threads, preventing the passage of any but 

 the smallest organisms. In the interior is an elaborate apparatus 

 for filtering out the minute organisms from the water as it passes 

 through. In Appendicularia and Kowalevskia the house also 

 encloses the animal : in Fritillaria it does not do so. 



Among the simple Ascidians there is a considerable degree of 

 uniformity of structure, and little need be added here to the 

 account given of the example. The shape varies a good deal : it 

 is sometimes cylindrical, sometimes globular, sometimes com- 

 pressed ; usually sessile and attached by a broad base, often with 

 root-like processes, but in other cases (e.g. Boltenia) elevated on 



a longer or shorter stalk. 



peri.bd negH,' Most are solitary ; but 



some (the so-called social 

 Ascidians) multiply by 

 budding, stolons being 

 given off on which new 

 zooids are developed, 

 so that associations or 

 colonies are formed ; 

 but the connection be- 

 tween the zooids is not 

 close, and their tests 

 remain distinct and 

 separate. The test varies 



FIG. 732. Diagram of Appendicularia from the right 



side. an. anus; ht. heart; int. intestine; ne. nerve; considerably in COnSlS- 

 ne'. caudal portion of nerve ; ne. gn.' principal nerve- , v. ' ^U 

 ganglion ; ne. gn", ne. gn." first two ganglia of nerve of tency, being Sometimes 

 tail ; noto. notochord ; ces. oesophagus : or. ap. oral aper- oU^^of n-plafirmnq tra na- 

 ture ; oto. otocyst (statocyst); peri. bd. peripharyngeal almost geiatl "US T 

 band ; ph. pharynx ; tes. testis ; stic/. one of the stigmata ; parent Or translucent, 

 stom. stomach. (After Herdman.) . , 



sometimes tougn and 



leathery, occasionally hardened by encrusting sand-grains or frag- 

 ments of shells, or by spicules of carbonate of lime. Calcareous 

 spicules may be developed in the substance of the mantle. 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 organs, in the position which 

 it occupies with regard to the various parts of the alimentary 

 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. 734, lang.}. 



In the composite Ascidians the zooids are embedded in a common 

 gelatinous mass formed of their united tests. The gelatinous colony 

 thus formed is sometimes flat and encrusting, sometimes branched 



rz&to 



