186 



TUNICATA 



Julin, 



have of late years been the subject of several very import- 

 ant memoirs. The researches of Todaro, Brooks (-?j), 

 Salensky (26), and others have elucidated the embryology, 

 the gemmation, and the life-history of the Salpidte ; and 

 Grobben, Barrels (27), and more especially Uljanin (28) 

 have elaborately worked out the structure and the details 

 of the complicated life-history of the Ddiolidx. Finally, 

 in an important work published in 1886 on the morpho- 

 Van logy of the Tunicata, E. van Beneden and Julin (jo) have, 

 Beneden mainly as the result of a close comparison of the embryo- 

 j gy O f Ascidians with that of Amphioxus and other 

 Chordata, added considerably to our knowledge of the 

 position and affinities of the Tunicata, and of the exact 

 relations of their organs to the corresponding parts of the 

 body in the Vertebrata. 



ANATOMY. 



Ascidia As a type of the Tunicata, Ascidia mentula, one of the 

 mmiula. i ar g er species of the Simple Ascidians, may be taken. This 

 species is found in most of the European seas, generally in 

 External shallow water on a muddy bottom. It has an irregularly 

 charac- ova te form, and is of a dull grey colour. It is attached to 

 ters ' some foreign object by one end (fig. 1). The opposite end 

 of the body is usually nar- 

 row, and it has a terminal 

 opening surrounded by eight 

 rounded lobes. This is the 

 mouth or branchial aperture, 

 and it always indicates the 

 anterior end J of the animal. 

 About half-way back from 

 the anterior end, and on a 

 rounded projection, is the 

 atrial or cloacal aperture 

 an opening surrounded by 

 six lobes which is always 

 placed upon the dorsal re- 

 gion. When the Ascidian 

 is living and undisturbed, 

 water is being constantly 

 drawn in through the 

 branchial aperture and 

 passed out through the 

 atrial. If coloured par- 

 ticles be placed in the 

 water near the apertures, 

 they are seen to be sucked 

 into the body through the 

 branchial aperture, and after 

 a short time some of them 

 are ejected with consider- 

 able force through the atrial 

 aperture. The current of 

 water passing in is for re- 

 spiratory purposes, and it 

 also conveys food into the animal. The atrial current is 

 mainly the water which has been used in respiration, but 

 it also contains all excretions from the body, and at times 

 the ova and spermatozoa or the embryos. 



The test. The outer grey part of the body, which is attached at 

 or near its posterior end and penetrated by the two aper- 

 tures, is the "test." This is a firm gelatinous cuticular 

 secretion from the outer surface of the ectoderm, which is 

 a layer of flat cells lining its inner surface. Although at 

 first produced as a cuticle, the test soon becomes organized 

 by the migration into it of cells derived from the ectoderm 

 (see fig. 2). These test cells may remain as rounded or 

 fusiform or stellate cells imbedded in the gelatinous matrix, 

 to which they are constantly adding by secretions on their 



1 Some writers use a different nomenclature of regions ; see (77). 



surfaces ; or they may develop vacuoles in their proto- 

 plasm, which become larger and fuse to form a huge ovate 



[0. 1. Asclilin mcntulu from the right 

 side, at, atrial aperture ; br, branchial 

 aperture ; t, test. (Original.) 



m, mantle ; e, ectoderm ; (c, 



, tm, matrix ; We, bladder cell ; s, s", blood sinus 



lime SO that in mantle being drawn out into test ; me, mantle cells ; 

 i y. septum of vessel. (From Henlman, Challenger Report. 



one or several 



of them together produce a calcareous spicule in the test. 

 Only the unmodified test cells and the bladder cells are 

 found in Ascidia mentula. Calcareous spicules are found 

 chiefly in the Didemnidse, amongst Compound Ascidians ; 

 but pigmented cells may occur in the test of almost all 

 groups of Tunicata. The matrix in which these structures 

 are imbedded is usually clear and apparently homogeneous; 

 but in some cases it becomes finely fibrillated, especially 

 in the family Cynthiidse. It is this matrix which contains 

 tunicine. At one point on the left side near the posterior 

 end a tube enters the test, and then splits up into a num- 

 ber of branches, which extend in all directions and finally 

 ' terminate in rounded enlargements or bulbs, situated chiefly 

 in the outer layer of the 

 test. These tubes are 

 known as the " vessels " of 

 the test, and they contain 

 blood. Each vessel is 

 bounded by a layer of ec- 

 toderm cells lined by con- 

 nective tissue (fig. 3, .B), 

 and is divided into two 



tubes by a septum of con- s~ ''ft 



nective tissue. The septum Flo s __ At a vcssel f rom the test. B.dia- 



does not extend into the grammatic transverse section of a ves- 



, gel. ec, ectoderm ; ct, connective tissue ; 



terminal bulb, and conse- S) ^ the two tubes ; y, septum ; (fc, ter- 



quently the two tubes com- "" al bulb - (" ri a' a1 -) 

 municate at their ends (fig. 3, A). The vessels are formed 

 by an outgrowth of a blood sinus (derived originally from 

 the blastoccele of the embryo) from the body wall (mantle) 

 into the test, the wall of the sinus being formed by con- 

 nective tissue and pushing out a covering of ectoderm in 

 front of it (fig. 2, '). The test is turned inwards at the 

 branchial and atrial apertures to line two funnel-like tubes, 

 the branchial siphon leading to the branchial sac and the 

 atrial siphon leading to the atrial or peribranchial cavity. 



The body wall, inside the test and the ectoderm, is formed Mantle, 

 of a layer (the somatic layer of mesoderm) of connective 

 tissue, inclosing muscle fibres, blood sinuses, and nerves. 

 This layer (the mantle) has very much the shape of the test 

 outside it, but at the two apertures it is drawn out to form 

 the branchial and atrial siphons (fig. 4). In the walls of 

 these siphons the muscle fibres form powerful circular 

 bands, the sphincter muscles. Throughout the rest of the 

 mantle the bands of muscle fibres form a rude irregular 

 network. They are numerous on the right side of the body, 

 and almost totally absent on the left. The muscles are all 

 formed of very long fusiform non-striped fibres. The con- 

 nective tissue of the mantle is chiefly a clear gelatinous 



