TUNICATA. 



1211 



(A. Intestmalis, &c.) the exterior layer is much 

 the thickest, and is composed of about twelve 

 distinct fascicles, the fibres of which are 

 somewhat separated from one another, insensi- 

 bly disappearing on the anterior and superior 

 borders. In others (Cynthia niicrocosmus, &c.), 

 this external layer is very slight, and formed 

 of distinct fibres, whilst the internal fibres are 

 stronger, transversely oblique, and interlacing. 

 Circular bands of muscles generally surround 

 the tube of each orifice, and are rather 

 stronger towards its base. The fibres forming 

 these collars of the two tubes occasionally in- 

 terlace with each other at the interval between 

 the tubes, in a figure-of-eight pattern. Some- 

 times these muscular collars or sphincters are 

 very indistinct, and not so apparent as another 

 set of fascicles, which run in a diagonally 

 transverse direction across the sac, and pass 

 up the sides of the tubes, converging at the 

 orifices. The latter bands diverge from the 

 two orifices in two sets, which in part cross 

 each other obliquely, and form an open net- 

 work. 



In Chelyosoma (fig.7T7.a,b) each of the six 

 triangular valvules that surround either orifice 

 is furnished with a fan-shaped set of muscular 

 fibres, adhering at one end to the inner sur- 

 face of the test, and at the other extremity to 

 a small papillary process on the valvule. Be- 

 sides this set of muscles, and within them, is 

 another set which passes laterally from one 

 papilla to another, forming a sphincter, the 

 base of which is hexagonal. There are other 

 strong subcutaneous muscular fibres passing 

 from the edge of the upper part of the tunic 

 to that of the lower, and also from the edge of 

 each of the coriaceous plates forming the 

 upper surface (c, c). 



By the action of these muscles, the body or 

 inner sac of the animal is transversely con- 

 tracted and somewhat lengthened; the tubular 

 processes also of the mantle are closed, and 

 more or less retracted. When the animal is 

 alarmed, the contraction takes place rapidly, 

 causing the water contained in the respiratory 

 and cloacal cavities to be ejected by one or 

 other of the orifices to the height of even 

 three feet. 



There is considerable analogy between the 

 muscular system of the Ascidits and that of 

 the Bryozoa. In the latter there are retractor 

 muscles of the sheath and the intestinal tube ; 

 and in the interior of the cells there exist also 

 transverse oblique cords, traversing the peri- 

 intestinal cavity, and attached to one or the 

 other surface of the skin. These transverse 

 muscles contribute to open the tentacular 

 circlet; and in the Ascidi<z,by their contraction, 

 produce the jet that escapes from each of the 

 two apertures. 



Embryo- genesis of the Simple As cidians. In 

 examining under the microscope a portion of 

 the ovary ofavecentAscidia, eggs in all stages 

 of development may often be observed. In 

 describing the various conditions and modifi- 

 cations of the ova of this family in course of 

 development, we shall borrow largely from the 

 careful and extensive researches given to the 



world by Professor Van Beneden, to whom 

 naturalists are so deeply indebted for much 

 valuable information on the subject of the em- 

 bryo-genesis of many of the Polypifera* and 

 other groups of animals. The earliest form of 

 the ovum is a simple vesicle; it next appears 

 as a vesicle with an inner vesicle, which is 

 evidently "the vesicle of Purkinje;" and, 

 thirdly, with a second, still smaller vesicle, 

 " the vesicle of Wagner ;" these latter vesicles, 

 one within the other, being enveloped in the 

 outer or vitelline membrane (fig. 781. A). 



The space between the external membrane 

 and Purkinje's vesicle is occupied by a sub- 

 stance, at first clear and probably fluid, but 

 soon appearing granulated, constituting the 

 vitellus. This yolk increases rapidly, soon oc- 

 cupying the whole ovum : at the periphery its 

 granules or cells become organised (Jig. 781. 

 B), uniting among themselves, and forming a 

 continuous sacciform membrane. The mode of 

 growth of the vitellus is by new cells being 

 formed in the interior of the large older cells, 

 and subsequently producing others in their 

 own interior. The growth of the whole ovum 

 is evidently carried on by this process. 



The whole surface of the vitellus soon after- 

 wards becomes embossed, presenting a mul- 

 berry-like appearance (fig. 781. c). The ger- 

 minal vesicles disappear. At the centre of 

 each little mammilla of the surface a trans- 

 parent vesicle becomes distinctly visible, and 

 the whole appear like so many ova each with 

 the vesicle of Purkinje. These mammillae in- 

 crease, and their peripheral substance rapidly 

 becomes granulated, in a similar manner to 

 the change that took place in the whole vi- 

 tellus. The mammillated appearance is now 

 soon lost from the growth of the granules, 

 and the surface becomes uniform, the mam- 

 millae of the periphery uniting into a mem- 

 brane, which constitutes the blastoderm. Ac- 

 cording to Milne-Edwards, the blastoderm in 

 the BotryllidfB is formed at a determinate point 

 only ; but in Cynthia ampulla (the species in 

 which Van Beneden watched the modifica- 

 tions of the ovum) the latter observer feels 

 assured that it is formed simultaneously all 

 over the yolk, as in all the inferior animals. 

 At this period a number of vesicles become 

 somewhat regularly arranged over the exterior 

 of the ovum, which by their union with one 

 another form a new enveloping membrane. 

 Beneath this membrane a space, occupied by 

 fluid, soon becomes apparent. All over the 

 exterior of this new membrane are distributed 

 white, transparent vesicles, having much the 

 appearance of oil globules. This envelope 

 soon becomes more and more extended by 

 the increase of the contained albuminous fluid, 

 and a second membrane appears beneath it, 

 on the surface of which also oil-like globules 

 become apparent, as on the external mem- 

 brane (fig. 781. D). The ovum is now com- 

 posed of an external membrane, of a white 



* See POLYPIFERA ; also Papers by Prof. Van 

 Beneden on Campanularia, Tubularia, and Bryozoa, 

 Me'm. Brux. Acad. torn. xvii. xviii. and xix. 



