TUNICATA 



191 



Thali- 

 acea. 



Charac- 

 ters of 

 Cydo- 

 myaria. 



Struc- 

 ture of 

 Dolio- 

 lum. 



If 



points more or less anteriorly. It shows distinct traces of aaeta- 

 meric segmentation, having its muscle bands broken up into myo- 

 tomes, while the nerve cord presents a series of enlargements from 

 which distributary nerves are given off (fig. 12, ng"). Near 

 the base of the tail there is a distinct elongated ganglion 

 (fig. 12, n<f). The anterior (cerebral) ganglion has connected 

 with it an otocyst, a pigment spot, and a tubular process 

 opening into the branchial sac and representing the dorsal 

 tubercle and associated parts of an ordinary Ascidian. The 

 branchial aperture or mouth leads into the branchial sac or 

 pharynx. There are no tentacles. The endostyle is short. 

 There is no dorsal lamina, and the peripharyngeal bands run 

 dorsally and posteriorly. The wall of the branchial sac has * ' 

 only two ciliated apertures. They are homologous with the 

 primary stigmata of the typical Ascidians and the gill 

 clefts of Vertebrates. They are placed far back on the -ven- 

 tral surface, one on each side of the middle line, and lead 

 into short funnel-shaped tubes which open on the surface of 

 the body behind the anus (fig. 12, at). These tubes corre- 

 spond to the right and left atrial involutions which, in an 

 ordinary Ascidian, fuse to form the peribranchial cavity. The 

 heart, according to Lankester, is formed of two cells, which 

 are placed at the opposite ends and connected by delicate con- 

 tractile protoplasmic fibrils. The large ovary and testis are placed 

 at the posterior end of the body. The remainder of the structural 

 details can be made out from fig. 12. 



The family Appcnditulariidx comprises the genera, Oikoplfura 

 (Mertens), and Appendicularia (Cham.), in both which the body is 

 short and compact and the tail relatively long, while the endostyle 

 is straight ; FrUHlaria (Q. and G.), in which the body is long and 

 composed of anterior and posterior regions, the tail relatively short, 

 the endostyle recurved, and an ectodermal hood is formed over the 

 front of the body ; and KouxUevsJcia (Fol), a remarkable form de- 

 scribed by Fol (14), in which the heart, endostyle, and intestine 

 are said to be absent, while the branchial sac is provided with four 

 rows of ciliated tooth-like processes. 



Order II. THALIACEA. 



Free-swimming pelagic forms which may be either simple or 

 compound, and the adult of which is never provided with a tail or a 

 notochord. The test is permanent and may be either well developed 

 or very slight. The musculature of the mantle is in the form of 

 more or less complete circular bands, by the contraction of which 

 locomotion is effected. The branchial sac has either two large or 

 many small apertures, leading to a single peribranchial cavity, into 

 which the anus opens. Alternation of generations occurs in the life- 

 history, and may be complicated by polymorphism. The Thaliacca 

 comprises two groups, Cyclomyaria and Hemimyaria. 



Sub-order 1. Cyclomyaria, 



Free-swimming pelagic forms which exhibit alternation of genera- 

 tions in their life-history but never form permanent colonies. The 

 body is cask -shaped, with the branchial and atrial apertures at the 

 opposite ends. The test is more or less well developed. The 

 mantle has its musculature in the form of circular bands surrounding 

 the body. The branchial sac is fairly large, occupying the anterior 

 half or more of the body. Stigmata are usually present in its 

 posterior part only. The peribranchial cavity is mainly posterior 

 to the branchial sac. The alimentary canal is placed ventrally 

 close to the posterior end of the branchial sac. Hermaphrodite 

 reproductive organs are placed ventrally near the intestine. 



This group forms one family, the DOLIOLID.E, including two 

 genera, Doliolum (Quoy and Gaimard) and Anchinia (C. Vogt). 



Doliolum, of which several species are known from various 

 seas, has a cask-shaped body, usually from 1 to 2 cm. in length. 

 The terminal branchial and atrial apertures (fig. 13) are lobed, 

 and the lobes are provided with sense organs. The test is very 

 slightly developed and contains no cells. The mantle has eight or 

 nine circular muscle bands surrounding the body. The most 

 anterior and posterior of these form the branchial and atrial 

 sphincters. The wide branchial and atrial apertures lead into 

 large branchial and peribranchial cavities, separated by the pos- 

 terior wall of the branchial sac, which is pierced by stigmata ; con- 

 sequently there is a free passage for the water through the body 

 along its long axis, and the animal swims by contracting its ring- 

 like muscle-bands, so as to force out the contained water posteriorly. 

 Stigmata may also be found on the lateral walls of the branchial 

 sac, and in that case there are corresponding anteriorly directed 

 diverticula of the peribranchial cavity. There is a distinct endo- 

 style on the ventral edge of the branchial sac and a peripharyngeal 

 tand surrounding its anterior end, but there is no representative 

 of the dorsal lamina on its dorsal edge. The trsophagus com- 

 mences rather on the ventral edge of the posterior end of the 

 branchial sic, and runs backwards to open into the stomach, which 

 is followed by a curved intestine opening into the peribranchial 

 cavity. The alimentary canal as a whole is to the right of the 

 middle line. The hermaphrodite reproductive organs are to the 

 left of the middle line alongside the alimentary canal. They open 



into the peribranchial cavity. The ovary is nearly spherical, while 

 the testis is elongated, and may be continued anteriorly for a long 

 distance. The heart is placed in the middle line ventrally, be- 



at 



at I 



trs 



Fro. IS. Doliolvm dfntievlatMm, sexual generation, from the left side. Letter- 

 ing as for fig. 12 ; m 1 m8, muscle Lands ; ng, nerve ganglion ; sg, stigmata ; 

 tgt, subneural gland : pbr, peribranchial cavity ; all, atrial lobes ; to, sense 

 organs ; brl, branchial lobes. (Original.) 



tween the posterior end of the endostyle and the cesophageal aperture. 

 The nerve ganglion lies about the middle of the dorsal edge of the 

 body, and gives off many nerves. Under it is placed the subneural 

 gland, the duct of which runs forward and opens into the anterior 

 end of the branchial sac by a simple aperture, surrounded by the 

 spirally twisted dorsal end of the peripharyngeal band (fig. 13, dt). 



The ova of the sexual generation produce tailed larvae ; these Develop- 

 develop into forms known as " nurses " (blastozooids), which are ment of 

 asexual, and are characterized by the possession of nine muscle Dolio- 

 bands, an auditory sac on the left side of the body, a ventrally- {urn. 

 placed stolon near the heart, upon which buds are produced, and 

 a dorsal outgrowth near the posterior end of the body. The buds 

 give rise eventually to the sexual generation, which is polymor- 

 phous, having three distinct forms, in two of which the reproduc- 

 tive organs remain undeveloped. The buds while still very young 

 migrate from their place of origin on the stolon, divide by fission, 

 and become attached to the dorsal outgrowth of the body of the 

 nurse, where they develop. The three forms produced are as follows. 

 (1) Nutritive forms (trophozooids), which remain permanently at- 

 tached to the nurse and serve to provide it with food ; they have 

 the body elongated dorso- ventrally, and the musculature is very 

 slightly developed. (2) Foster forms (phorozooids), which, like the 

 preceding, do not become sexually mature, but, unlike them, are 

 set free as cask-shaped bodies with eight muscle bands and a ventral 

 outgrowth, which is formed of the stalk by which the body was 

 formerly united to the nurse. On this outgrowth the (3) forms 

 (gonozooids) which become sexually mature are attached while still 

 young buds, and after the foster forms are set free these reproductive 

 forms gradually attain their complete development, and are event- 

 ually set free and lose all trace of their connexion with the foster 

 forms. They resemble the foster forms in having a cask-shaped 

 body with eight muscle bands, but differ in having no outgrowth 

 or process, and in having the reproductive organs Mly developed. 1 



Anchinia, of which only one species is known, A. rubra, from Anchinia. 

 the Mediterranean, has the sexual forms permanently attached 

 to portions of the dorsal outgrowth from the body of the unknown 

 nurse. The body is elongated dorso- ventrally. The test is well 

 developed and contains branched cells. The musculature is not 

 so well developed as in Doliolum. There are two circular bands 

 at the anterior end and two at the posterior, and two on the 

 middle of the body. The stigmata are confined to the obliquely 

 placed posterior end of the branchial sac. The alimentary canal 

 forms a U-shaped curve. The reproductive organs are placed on 

 the right side of the body. The life-history is still imperfectly 

 known. As in the case of Doliolum the sexual generation is 

 polymorphous, and has three forms, two of which remain in a 

 rudimentary condition so far as the reproductive organs are con- 

 cerned. In Anthinia, however, the three forms do not occur to- 

 gether on one stolon or outgrowth, but are produced successively, 

 the reproductive forms of the sexual generation being independent 

 of the " foster forms " (see Barrois, 27}. 



Sub-order 2. Hemimyaria, 



Free-swimming pelagic forms which exhibit alternation of genera- Charac- 

 tions in their life-history and in the sexual condition form colonies, ters of 

 The body is more or less fusiform, with the long axis antero-posterior, Hemi- 

 and the branchial and atrial apertures nearly terminal. The test myaria. 

 is well developed. The musculature of the mantle is in the form 

 of a series of transversely -running bands, which do not form com- 

 plete independent rings as in the Cyclomyaria. The branchial and 



1 For further details see Cljaniu (iS). 



