1194 



TUNICATA. 



together adapted to maintain a permanent 

 attachment to a fixed body. 



The external envelope of the Ascidiadts is 

 always perforated by the two apertures 

 characteristic of the Tunicnta, and analogous 

 to the prolonged respiratory orifices of the 

 CardktceoE and other Acephala. One of these 

 apertures*, nearly always placed at the .sum- 

 mit of the test, receives the sea-water, and 

 admits it into the branchial cavity. The 

 second aperture f is placed a little lower 

 than the first, and is in communication with 

 the rectum and oviduct. In Boltenia the ori- 

 fices are lateral ; in Cystingia the branchial 

 orifice is lateral, and the anal terminal ; the oral 

 orifice being always the highest in relative 

 position, and nearest to the insertion of the 

 pedicle by which the animal is suspended. In 

 Chclyosoma and Dendrodoa the orifices are 

 placed on the same plane ; in the former, on 

 the nearly flat superior surface of the animal, 

 and in the latter they are terminal. 



The edges of the orifices are more or less 

 crenulate or divided. The branchial orifice 

 is 8-9 rayed, and the anal 6 rayed, in Plial- 

 iusia ; both orifices are 4-fid in Cynthia, Bol- 

 tenia, and Dendrodoa. When contracted, they 

 are thrown into longitudinal folds ; this is 

 the more observable when the orifices are 

 somewhat prolonged into tubes. 



The test receives from the body blood- 

 vessels, which its semitransparency in some 

 species allows the eye to follow to the extreme 

 ramifications. In the thinner tests the vessels 

 are but few, and sometimes altogether escape 

 observation ; but in the thick pellucid test 

 of Ascidia mammillata the eye can discern an 

 extensive network of vascular ramifications. 

 The bloodvessels enter the test near the base. 

 The internal surface of the test has often a 

 glistening and pearly appearance, and is always 

 lined with an epithelium. In Boltenia reni- 

 formis this is a loose tissue, and forms a slight 

 attachment to the external surface of the mus- 

 cular sac or mantle of the body. 



In A. innmmillata a thin vertical septum 

 traverses a part of the cavity of the test. 



The test of the ClavclUmdce is very similar 

 in consistence and general appearance to that 

 of some of the more delicate species of the 

 AscidiadcB, but differs materially in having tu- 

 bular prolongations running from its base, 

 which are traversed by vessels continuous from 

 one individual to another, and from which 

 root-like processes young individuals are 

 continually being produced by the process of 

 gemmiparous reproduction (fig. 768.). 



The tube-like test of the Salpida: is " semicar- 

 tilaginous, or gelatinous, seeming as if carved 

 in crystal," its transparency rendering conspi- 

 cuous the brilliant hues of the liver of the 

 contained animal. It is usually more or less 

 angular, bearing elongated crests, denticles, 

 and other processes, by means of which the 

 cohesion of the aggregated individuals is 

 maintained (fig. 772.). 



* The first, oral, buccal, branchial, or respiratory, 

 f The second, anal, ventral, or the funnel. 



The tests of the little animals of which the 

 cylindrical body of the Pyrusoma is composed, 

 are intimately connected by their lateral sur- 

 faces, leaving their terminal orifices free (fig. 

 786. A). The tissue of the whole common 

 envelope is semicartilaginoiis, transparent, 

 tinted with azure and other colours. 



In the liotryllidcE the external tunic is re- 

 presented by the tegumentary tissue com- 

 mon to the whole association of animals, 

 and which may perhaps be compared to the 

 polypary of aggregated Polyps. The close 

 aggregation of the animals causes a fusion, 

 as it were, of the tests of the whole into 

 a coriaceous or gelatinous mass, coated with 

 a tough epidermic membrane, and more 

 or less regularly beset with individuals or 

 groups of individuals (systems) ; or, rather, 

 the soft test of the originally isolated indivi- 

 dual, or single system, increasing in bulk and 

 producing within itself more and more embryos 

 of individuals and systems, becomes an ex- 

 tended, homogeneous mass, in which nu- 

 merous individuals are lodged (fig. 771.). This 

 mass is irregular, fungous, jelly-like, spongy, 

 or coriaceous, incrusting other sessile Tuni- 

 cates and a variety of marine bodies. 



Structure and chemical composition of the test. 

 We are indebted to the labours of MM. L6- 

 wig and Kolliker* for considerable information 

 on the subject of the constituent elements 

 and the growth of the test of the simple and 

 the compound Ascidians. These researches 

 were undertaken with the view of examining 

 into Dr. Schmidt's statement of the existence 

 of the vegetable element " cellulose" in the 

 tissue of the ascidian envelope. This they 

 found to be correct (as the following state- 

 ments will show), and they have also offered 

 a lucid explanation of the fact, to which we 

 shall subsequently refer. Cellulose forms the 

 cell walls of vegetables, and is unaffected either 

 by soda or hydrochloric acid. 



The cartilaginous envelope of Phallusia 

 mammillaris (Fam. Ascidiadte), examined in 

 specimens preserved in spirit, is composed of 

 three layers of different thicknesses (fig. 773.). 

 The internal layer, formed simply of polygonal, 

 nucleated, epithelial cells, measuring 0'005'", 

 covers all the interior surface of the test ; at 

 the two external orifices, and at the points 

 where it receives the nutrient vessels of the 

 test, it is united with another epithelial tissue 

 covering the mantle. The second layer is con- 

 siderably thicker, and is composed of a homo- 

 geneous substance, containing crystals and 

 nuclei. The former are not present every- 

 where, and are, perhaps, quite absent in the 

 recent animals ; when present they are visible 

 to the naked eye, and appear like white stria; ; 

 seen under a moderately magnifying power, 

 they have the form of crystals united in the 

 form of a star, or of irregular and polymor- 

 phous concretions. The nuclei are present in 

 considerable numbers and under different 

 forms ; those situated towards the interior are 

 round, O'OOlo'" 0'002'", with one or two 



* Annales des Sciences Nature-lies, 3 ser. torn. vi. 1846. 



