The sea-squirts present no exceptions to the universal rule of Harvey, _omne animal ex ovo_—this philosopher believing in the universal development of the animal-form from an ovum or egg. But unlike most higher animals, the young sea-squirt does not come from the egg in the likeness of the parent. It first appears as a tadpole-like body, this creature—the larva as it is named—being produced in some thirty hours after the development of the egg begins. The head of the tadpole is provided with pigment spots or rudimentary eyes, and with three little suckers, by means of which it ultimately attaches itself to fixed bodies, prior to assuming the form of the adult and perfect Ascidian. The tail of the tadpole-larva next becomes retracted within its body, and therein disappears, whilst after fixing itself, the well-known features of the sea-squirt become duly developed. A Russian zoologist has remarked that in the tail of the sea-squirt a long rod-like body is to be seen. Now in the lowest fishes, the spine exists in a similar and rod-like condition; and hence, by a certain school of naturalists, it is urged that the vertebrates may have originated in the past from some form resembling the sea-squirt larva, in whose tail we are therefore invited to behold the first stage of the vertebrate backbone or spine. It is noteworthy to observe, however, that the opinions of these naturalists are by no means accepted by the scientific world at large; and one eminent German observer declared that the rod-like body in the sea-squirt larva’s tail was not situated in the back, but in the opposite region of the body, and that therefore it could not be regarded as corresponding to the ‘back’-bone of higher animals. Certain near relations of the sea-squirt become of exceeding interest from their departure from the more usual and staid type of Ascidian structure. Amongst these errant members of the sea-squirt tribe the most remarkable perhaps are the _Salpæ_—clear, gelatinous animals, met with swimming in long connected chains on the surface of the sea in tropical regions. The celebrated novelist Chamisso, author of the charming story, _Peter Schlemil or the Shadowless Man_, who to his literary tastes united a striking aptitude for natural history research, discovered that the young of these chain-salpæ invariably appears as a _single_ form; whilst each single salpa has the power of producing a connected chain. Thus the salpa sea-squirts exist in two distinct forms—chain-salpas and single salpas, and to use Chamisso’s own words: ‘A salpa mother is not like its daughter or its own mother, but resembles its grand-daughter and its grand-mother.’