TUNIOATA 



189 



ventral edge of the branchial sac externally to the endostyle, 

 and communicates laterally with the ventral ends of all the 

 transverse vessels of the branchial sac. The sinus leaving 

 the dorsal end of the heart is called the cardio-visceral 

 vessel, and this, after giving off to the test the branch 

 above mentioned, breaks up into a number of sinuses, 

 which ramify over the alimentary canal and the other 

 viscera. These visceral lacunae finally communicate with 

 a third great sinus, the viscero-branchial vessel, which runs 

 forward along the dorsal edge of the branchial sac exter- 

 nally to the dorsal lamina and joins the dorsal ends of all 

 the transverse vessels of the branchial sac. Besides these 

 three chief systems there are numerous lacunse in all parts 

 oi the body, by means of which anastomoses are established 

 between the different currents of blood. All these blood 

 spaces and lacunse are to be regarded as derived from the 

 blastocoele of the embryo, and not, as has been usually 

 Course of supposed, from the coelom (jo). When the heart contracts 

 circula- ventro-dorsally, the course of the circulation is as follows : 

 the blood which is flowing through the vessels of' the 

 branchial sac is collected in an oxygenated condition in 

 - the branchio-cardiac vessel, and, after receiving a stream 

 of blood from 

 the test, en- 

 ters the 

 heart. It is 

 then pro- 

 pelled from 

 the dorsal 

 end of 

 heart 



tion, 



br- 



■•vt 



the Fig. 9.— Diagram of oiroulation in Ascidia. Lettering as te- 

 . . fore; &c, branchio-cardiac or ventral vessel; cv, cardio- 

 mtO visceral vessels; vb, viscero-branchial or dorsal vessel; 

 the cardio- ^'j vessels to test. (Original.) 



visceral vessels, and so reaches the test and digestive and 

 other organs ; then, after circulating in the visceral lacunae, 

 it passes into the viscero-branchial vessel in^ an impure 

 condition, and is distributed to the branchial vessels to be 

 purified again. When the heart on the other hand contracts 

 dorso-ventrally, this course of circulation is reversed. As 

 the test receives a branch from each end of the heart, it 

 follows that it has afferent and efferent vessels which- 

 ever way the blood is flowing. In some Ascidians the 

 vessels in the test become very numerous and their end 

 branches terminate in swollen bulbs close under the outer 

 surface of the test. In this way an accessory respiratory 

 organ ^ is probably formed in the superficial layer of the 

 test. The blood corpuscles are chiefly colourless and 

 amoeboid ; but in most if not all Ascidians there are also 

 some pigmented corpuscles in the blood. These are gener- 

 ally of an orange or reddish brown tint, but may be opaque 

 white, dark indigo-blue, or of intermediate colours. Pre- 

 cisely similarly pigmented cells are found throughout the 

 connective tissue of the mantle and other parts of the body. 

 A. nientida is hermaphrodite, and the reproductive organs 

 lie, with the alimentary canal, on the left side of the body. 

 The ovary is a ramified gland which occupies the greater 

 part of the intestinal loop (see fig. 4). It contains a cavity 

 which, along with the cavities of the testis, is derived from 

 a part of the original coelom, and the ova are formed from 

 its walls and fall when mature into the cavity. The 

 oviduct is continuous with the cavity of the ovary and 

 leads forwards alongside the rectum, finally opening near 

 the anus into the peribranchial cavity. The testis is com- 

 posed of a great number of delicate branched tubules, 

 which ramify over the ovary and the adjacent parts of the 

 intestinal wall. Those tubules terminate in ovate swell- 

 ings. Near the commencement of the rectum the larger 

 tubules unite to form the vas deferens, a tube of consider- 

 able size, which runs forwards alongside the rectum, and, 

 like the oviduct, terminates by opening into the peri- 

 ' See Herdman, Nature, vol. xxxi. p. 247. 



branchial cavity close to the anus. The lumen of the 

 tubules of the testis, like the cavity of the ovary, is a part 

 of the original ccelom, and the spermatozoa are formed 

 from the cells lining the wall. In some Ascidians repro- 

 ductive organs are present on both sides of the body, and 

 in others {Polycarpa) there are many complete sets of both 

 male and female systems, attached to the inner surface of 

 the mantle on both sides of the body and projecting into 

 the peribranchial cavity. ^ 



EmBETOLOOY ' AND LiFE HiSTOEY. 



In most Ascidians the eggs are fertilized in the peribranchial Emliryc- 

 cavity, and undergo most of their development before leaving the logy, 

 parent ; in some cases, however, the eggs are laid, and fertilization 

 takes place in the surrounding water. The segmentation is com- 

 plete and regular (fig. 10, A) and results in the formation of a 

 spherical blastula, which then undergoes invagination (fig. 10, B). 

 The embryo elongates, and the blastopore or invagination opening 

 comes to be placed on the dorsal edge near the posterior end (fig. 

 10, C). The hypoblast cells lining the archenteron are columnar 

 in form, while the epiblast cells are more cubical (fig. 10, B, C, D). 

 The dorsal surface of the embryo now becomes flattened and then 

 depressed to form a longitudinal groove, extending forwards from 

 the blastopore to near the front of the body. This "medullary 

 groove" now becomes converted into a closed canal by its side 

 walls growing up, arching over, and coalescing in the median dorsal 



Fig. 10.— Stages m the embryology of a Simple Ascidian (after Kowalevsky). 

 A to r. Longitudinal vertical sections of embryos, all placed with the dorsal 

 surface uppermost and the anterior end at the right. A. Early blastula 

 stage, during segmentation. B. Early gastrula stage. C. Stage after gas- 

 trula, showing commencement of notochord. D. LatSr stage, showing forma- 

 tion of notochord and of neural canal. B. Embryo showing body and tail 

 and completely formed neural canal. P. Larva just hatched ; end of tail 

 cut off. G. Transverse section of tail of larva. 



adp, adhering papillae of larva ; at, epiblastic (atrial) involution;. o«, auditory 

 organ of larva ; ur, archenteron ; ic, blastoccele ; ip, blastopore ; c%, noto- 

 chord ; ej), epiblast ; %y, hypoblast ; nc, neural canal ; mc, neurenterio 

 canal ; oc, ocular organ of larva ; g, gelatinous investment of embryo ; m, 

 muscle cells of tail ; mes, mesenteron ; mc, mesoderm cells ; nv, cerebral 

 vesicle at anterior end of neural canal. 



line (fig. 10, D). This union of the laminm dorsales to form the 

 neural canal commences at the posterior end behind the blastopore 

 and gradually extends forwards. Consequently the blastopore 

 comes to open into the posterior end of the neural canal (fig. 10, 

 D), while the anterior end of that cavity remains open to the 

 exterior. In this way the archenteron communicates indirectly 

 with the exterior. The short canal leading from the neural canal 

 to the archenteron is known as the neurenterio canal (fig. 10, 



^ For structure of other forms, see p. 614 sq. below. 

 ' For reproduction by gemmation, see under "Classification," p. 

 614 «2' below. 



