BIOLOGICAL NOTES

Abstract

THE MALE OF SALPA.—The development of the spermatozoa in the Salpæ has hitherto not been satisfactorily studied. Only two years ago Mr. Brooks, of Boston, stated that the testis developed from the elæoblast, and moreover maintained that of the two generations which alternate in these pelagic Tunieates—the one set, the “chain” Salpæ, are exclusively males, whilst the other set, the “solitary” Salpæ, are exclusively females. This view involved the theoretical assumption that the single egg which is found in every individual of a chain of Salpæ does not really belong to that individual which is only a male, and has the egg laid into it by the solitary Salpa from which the chain is derived by budding. Accordingly, the elæoblast in the solitary Salpa which, according to Brooks, is female, represents the testis and points to a primitive hermaphroditism; whilst in the chain Salpæ (actual males, according to Brooks), the elæoblast becomes testis. The more usual view is that the solitary Salpæ are not sexually differentiated at all, and that the chain Salpæ are hermaphrodites. Prof. Salensky, of Kasan, has recently published (Zeitsch. wiss. Zoologie, 1878, Supplement 2) some observations on this matter, having previously given a very careful account of the development of these organisms, and at the same time he enters into a discussion of the relationships of various Tunicata which has much interest. He shows that the eggs found in the chain Salpæ cannot be regarded as given off from the solitary mother into the budded chain because there is no specialised ovarian cord or rudiment in the proliferous mother. She differs in this respect from the adult proliferous persons in Pyrosoma—which really, as shown by Huxley and by Kowalewsky, give to their buds a part of their own ovarian rudiment. The solitary Salpa has nothing of the kind to give. Further, Salensky shows that the eteoblast has nothing to do with the testis. It exists in the solitary Salpa, and in the chain Salpa appears only for a brief period, and then disappears; but is certainly not developed into a testis. Accordingly the solitary Salpa is devoid of all trace of either ovary or testis. The elæoblast appears very probably, according to Salensky, to represent the noto-chord. The tailed larvæ of the true Ascidians possess a well-developed notochord, and present to us the ancestral form of the Tunicata, which only persists to the adult condition in the Appendiculariæ. In the Ascidiæ the tail and notochord atrophy as development advances. In Doliolum the young form which develops from the egg has a short tail with an axis apparently intermediate in character between the notochord of the Ascidian tadpole and the elæoblast of the Salpæ. The sexual form of Doliolum developed by budding from a second sexless generation, is devoid of tail. Salensky holds that we must distinguish in the Tunicata such simple budding from the adult as is presented in the Pyrosoma colonies and others, and that kind of budding which definitely characterises the alternation of generations morphologically distinguishable from one another. The sexless nurse, constituting the one generation, appears to retain the characteristics of the ancestral Tunicate form with tail and notochord. It corresponds to the Ascidian tadpole, and is represented in more or less completely modified condition by the tailed sexless nurses of Doliolum, by the solitary Salpæ, and by the Cyathozooid or primary person of the Pyrosoma colony, which gives rise to the colony by a process of budding which it is necessary to distinguish very widely from that which the persons of the colony exhibit at a later stage themselves. Just as the Ascidian tadpole becomes itself atrophied and meta morphosed so as to form the sexually mature Ascidian, so do the “nurses” above mentioned give rise by budding to a generation not possessing their own archaic characters, but bearing sexual organs and corresponding to the adult Ascidian, and thus we have an alternation of form in the successive gamic and agamic generations. Should multiplication by budding or fission be confined to the later sexual phase, then there is no morphological alternation of generations. Salensky thinks that a hopeful way of gaining a deeper insight into the phenomenon of true metagenesis lies in the further study of the cases pre sented by Tunicata.