The larva at this stage is represented in a side view in [fig. 8] IV. The epidermis is formed throughout of a single layer of cells. In the trunk the mesenteron is shewn at dd and the dilated part of the nervous system, no longer communicating with the exterior, at n. In the tail the notochord is shewn at ch, the muscles at m, and the solid remnant of the ventral wall of the archenteron at dd´. The delicate continuation of the neural canal in the tail is seen above the notochord at n. An optical section of the tail is shewn in [fig. 6]. It is worthy of notice that the notochord and muscles are formed in the same manner as in Amphioxus, except that the process is somewhat simplified. The mode of disappearance of the archenteric cavity in the tail, by the employment of the whole of its walls in the formation of various organs, is so peculiar, that I feel some hesitation in accepting Kowalevsky’s statements on this head[6].
Fig. 8. Various stages in the development of Phallusia mammillata. (From Huxley; after Kowalevsky.)
The embryos are represented in longitudinal vertical section.
I. Commencing gastrula stage. fh. segmentation cavity.
II. Late gastrula stage with flattened dorsal surface. eo. blastopore; ch. notochord; dd. hypoblast.
III. A more advanced embryo with a partially-formed neural tube. ch. and dd. as before; n. neural tube; c. epiblast.
IV. Older embryo in which the formation of the neural tube is completed. dd. hypoblast enclosing persistent section of alimentary tract; dd´. hypoblast in the tail; m. muscles.
V. Larva just hatched. The end of the tail is not represented. a. eye; gb. dilated extremity of neural tube with otolith projecting into it; Rg. anterior swelling of the spinal division of the neural tube; f. anterior pore of neural tube; Rm. posterior part of neural tube; o. mouth; Chs. notochord; kl. atrial invagination; dd. branchial region of alimentary tract; d. commencement of œsophagus and stomach; dd´. hypoblast in the tail; m. muscles; hp. papilla for attachment.
VI. Body and anterior part of the tail of a two days’ larva. klm. atrial aperture; en. endostyle; ks. branchial sack; 1ks. 2ks. branchial slits; bb. branchial vessel between them; ch. axial portion of notochord; chs. peripheral layer of cells. Other reference letters as before.
The larva continues to grow in length, and the tail becomes further curled round the ventral side of the body within the egg-membrane. Before the tail has nearly reached its full length the test becomes formed as a cuticular deposit of the epiblast cells (O. Hertwig, No. [13], Semper, No. [37]). It appears first in the tail and gradually extends till it forms a complete investment round both tail and trunk, and is at first totally devoid of cells. Shortly after the establishment of the test there grow out from the anterior end of the body three peculiar papillæ, developed as simple thickenings of the epidermis. At a later stage, after the hatching of the larva, these papillæ develop glands at their extremities, secreting a kind of glutinous fluid[7]. After these papillæ have become formed cells first make their appearance in the test; and there is simultaneously formed a fresh inner cuticular layer of the test, to which at first the cells are confined, though subsequently they are found in the outer layer also. On the appearance of cells in the test the latter must be regarded as a form, though a very abnormal one, of connective tissue. When the tail of the larva has reached a very considerable length the egg-membrane bursts, and the larva becomes free. The hatching takes place in Asc. canina about 48-60 hours after impregnation. The free larva ([fig. 8] V.) has a swollen trunk, and a very long tail, which soon becomes straightened out. It has a striking resemblance to a tadpole (vide [fig. 10]).
In the free larval condition the Ascidians have in many respects a higher organization than in the adult state. It is accordingly convenient to divide the subsequent development into two periods, the first embracing the stages from the condition represented in [fig. 8] V. up to the full development of the free larva, and the second the period from the full development of the larva to the attainment of the fixed adult condition.
Growth and Structure of the free larva.
The nervous system. The nervous system was left as a closed tube consisting of a dilated anterior division, and a narrow posterior one. The former may be spoken of as the brain, and the latter as the spinal cord; although the homologies of these two parts are quite uncertain. The anterior part of the spinal cord lying within the trunk dilates somewhat ([fig. 8] V. and VI. Rg) and there may thus be distinguished a trunk and a caudal section of the spinal cord.
