190 



TUNICATA 



D, nee). Previous to this stage some of the hypoblast cells at the 

 front edge of the blastopore and forming part of the dorsal wall of 

 the arehenteron (fig. 10, C, ch) have become separated off, and then 

 arranged to form an elongated band, two cells wide, underlying 

 the posterior half of the neural canal (fig. 10, D, E, ck. ). This 

 is the origin of the notochord. Outgrowths from the sides of the 

 arehenteron give rise to laterally placed masses of cells, which are 

 the origin of the mesoblast. These masses show no trace of meta- 

 meric segmentation. The cavities (reproductive and renal vesicles) 

 which are formed later in the mesoblast represent the ccelom. 

 Consequently the body-cavity of the Tunicata is a modified form 

 of enteroccele. The anterior part of the embryo, in front of the 

 notochord, now becomes enlarged to form the trunk, while the 

 posterior part elongates to form the tail (fig. 10, E). In the trunk 

 the anterior part of the arehenteron dilates to form the mescnteron, 

 the greater part of which becomes the branchial sac ; at the same 

 time the anterior part of the neural canal enlarges to form the 

 cerebral vesicle, and the opening to the exterior at the front end of 

 the canal now closes. In the tail part of the embryo the neural 

 canal remains as a narrow tube, while the remains of the wall of the 

 arehenteron the dorsal part of which becomes the notochord are 

 converted into lateral muscle bands (fig. 10, G) and a ventral cord 

 of cells, which eventually breaks up to form blood corpuscles. As 

 the tail grows longer, it becomes bent round the trunk of the embryo 

 inside the egg-membrane. About this period the epiblast cells 

 begin to form the test as a cuticular deposit upon their outer surface. 

 The test is at first devoid of cells and forms a delicate gelatinous 

 investment, but it shortly afterwards becomes cellular by the 

 migration into it of test cells formed by proliferation from the epi- 

 blast. 1 



Larval The embryo is hatched about two or three days after fertilization, 

 stage. in the form of a tadpole-like larva, which swims actively through 

 the sea by vibrating its long tail. The anterior end of the body 

 is provided with three adhering papill* (fig. 10, F, adp) in the 

 form of epiblastic thickenings. In the free-swimming tailed larva 

 the nervous system, formed from the walls of the neural canal, 

 becomes considerably differentiated. The anterior part of the 

 cerebral vesicle remains thin-walled (fig. 10, F), and two unpaired 

 sense organs develop from its wall and project into the cavity. 

 These are a dorsally ami posteriorly placed optic organ, provided 

 with retina, pigment layer, lens, and cornea, and a ventrally placed 

 auditory organ, consisting of a large spherical partially pigmented 

 otolith, attached by delicate hair-like processes to the summit of a 

 hollow crista acoustica (fig. 10, F, au). The posterior part of the 

 cerebral vesicle thickens to form a solid ganglionic mass traversed 

 by a narrow central canal. The wall of the neural canal behind the 

 cerebral vesicle becomes differentiated into an anterior thicker 

 region, placed in the posterior part of the trunk and having a 

 superficial layer of nerve fibres, and a posterior narrower part which 

 traverses the tail, lying on the dorsal surface of the notochord, and 

 gives off several pairs of nerves to the muscles of the tail. Just in 

 front of the anterior end of the nervous system a dorsal involution 

 of the epiblast breaks through into the upturned anterior end of 

 the mesenteron and thus forms the mouth opening. Along the 

 ventral edge of the mesenteron, which becomes the branchial sac, 

 the endostyle is formed as a narrow groove with thickened side 

 walls. It probably corresponds to the median portion of the thyroid 

 body of Vertebrata. A curved outgrowth from the posterior end 

 of the mesenteron forms the alimentary canal (oesophagus, stomach, 

 and intestine), which at first ends blindly. An anus is formed 

 later by the intestine opening into the left of two lateral epiblastic 

 involutions (the atria), which rapidly become larger and fuse dorsally 

 to form the peribranchial cavity. Outgrowths from the wall of the 

 branchial sac meet these epiblastic involutions and fuse with them 

 to give rise to the first formed pair of stigmata, which thus come 

 to open into the peribranchial cavity ; and these alone correspond 

 to the gill clefts of Amphioxus and the Vertebrata. 



Metamor. After a short free-swimming existence the fully developed tailed 

 phosis larva fixes itself by its anterior adhering papillre to some foreign 

 to adult object, and then undergoes a remarkable series of retrogressive 

 form. changes, which convert it into the adult Ascidian. The tail atro- 

 phios, until nothing is left but some fatty cells in the posterior 

 part of the trunk. The adhering papilla? disappear and are replaced 

 functionally by a growth of the test over neighbouring objects. 

 The nervous system with its sense organs atrophies until it is re- 

 duced to the single small ganglion, placed on the dorsal edge of the 

 pharynx, and a slight nerve cord running for some distance pos- 

 teriorly (Van Beneden and Julin). Slight changes in the shape 

 of the body and a further growth and differentiation of the branchial 

 sac, peribranchial cavity, and other organs now produce gradually 

 the structure found in the adult Ascidian. 



The most important points in connexion with this process of 

 development and metamorphosis are the following. (1) In the 

 Ascidiau embryo all the more important organs (e.g., notochord, 

 neural canal, arehenteron) are formed in essentially the same 



1 Some of the first test cells are also probably derived from the epithelium 

 of the egg follicle. 



manner as they are in Amphioxus and other Chordata. (2) The 

 free-swimming tailed larva possesses the essential characters of the 

 Chordala, inasmuch as it has a longitudinal skeletal axis (the noto- 

 chord) separating a dorsally placed nervous system (the neural 

 canal) from a ventral alimentary canal (the arehenteron) ; and 

 therefore during this period of its life-history the animal belongs 

 to the Chordata. (3) The Chordate larva is more highly organized 

 than the adult Ascidian, and therefore the changes by which the 

 latter is produced from the former may be regarded as a process of 

 degeneration (31). The important conclusion drawn from all this 

 is that the Tunicata are the degenerate descendants of a group of 

 the primitive Chordata (see below p. 618). 



CLASSIFICATION AND CHARACTERS OF GROUPS. 

 Order I. LAKVACEA. 



Free-swimming pelagic forms provided with a large locomotory Char- 

 appendage (the tail), in which there is a skeletal axis (the urochord). acters 

 A relatively large test (the "Haus") is formed with Larva 

 great rapidity as a secretion from the ectoderm ; it is 

 merely a temporary structure, which is cast off and 

 replaced by another. The branchial sac is simply an 

 enlarged pharnyx with two ventral ciliated openings 

 (stigmata) leading to the exterior. There is no se- 

 i. parate peribranchial cavity. The nervous 

 * system consists of a large dorsally placed 

 ganglion and a long nerve cord, which 

 stretches backwards over the alimentary 

 canal to reach the tail, along which it 

 runs on the left side of the 

 urochord. The anus opens 

 ventrally on the surface of 

 the body in front of the stig- 

 mata. No reproduction by 

 gemmation or metamorphosis 

 is known in the life-history. 

 This is one of the most in- 

 teresting groups of the Tuni- 

 cata, as it shows more com- 

 pletely than any of the rest 

 the characters of the original 

 ancestral forms. It has un- 

 dergone little or no degen- 

 eration, and consequently 

 corresponds more nearly to 

 the tailed -larval condition 

 than to the adult forms of 

 the other groups. The order 



FIG. ll.-Oikopleura copJtocerca in "Haus" includes a single family the 

 (after Fol), seen from right side, magnified APPENDICULARIIM), all the 

 six times. The arrows indicate the course members of which are minute 

 of the water ;x, lateral reticulated parts of and f re0 - swimming. They 



occur on the surface of the 



sea in most parts of the world. They possess the power to form Struc- 

 with great rapidity an enormously large investing gelatinous layer ture o 

 (fig. 11), which corresponds to the test of other groups. This was Appei 



p,.p n.a dicula 



n f<f ,y oi 



/At 



sir 



tu 



Fio. 12. Semi-diagrammatic view of Appendicularia from the right, a, anus ; 

 at, one of the atrial apertures ; app, tail ; br, branchial aperture ; brs, branchial 

 sac ; dt, dorsal tubercle ; end, endostyle ; h, heart ; i, intestine ; m, muscle 

 band of tail ; n, nerve cord in body ; ft', nerve cord in the tail ; o% cesophagus ; 

 ot, otocyst ; ov, ovary ; pp, perjpharyngeal band ; ijg, cerebral ganglion ; ng 1 , 

 caudal ganglion ; ng", enlargement of nerve cord in tail ; so, sense organ 

 (tactile) on lower lip ; sg, ciliated aperture iu pharynx ; st, stomach ; tes, tcstis ; 

 u, urochord ; u', its cut end. (Original.) 



first described by Von Mertens and by him named "Haus." It 

 is only loosely attached to the body and is frequently thrown off 

 soon after its formation. The tail in the Appendicvlariidee is at- 

 tached to the ventral surface of the body (fig. 12), and usually 



