By the term sea-squirts, the naturalist denominates some of the most remarkable animals which it is his province to study. In more polite phraseology the sea-squirts are termed ‘Ascidians,’ this appellation being derived from the Greek _askos_, meaning a wine-skin or Eastern leather-bottle, to which, in outward shape and form the sea-squirts bear a very close resemblance. And as a final designation, the animals under discussion may be known as ‘Tunicates,’ since their bodies are inclosed within a tough bag or ‘tunic,’ the chemical composition of which forms, as we shall presently shew, one of the notable points of their structure. The sea-squirts present themselves to the zoologist as a group of beings exhibiting many exceptions to the ordinary rules of animal organisation; and it may also be noted that they have attained a degree of scientific fame almost exceeding that which their most ardent admirers could have expected. The young sea-squirt has thus been credited in certain scientific speculations with presenting the naturalist and mankind at large with a _fac-simile_ of the early progenitor and far-back ancestor of the whole vertebrate group of animals, including man himself—in other words, it is maintained that the young sea-squirt, through some peculiar process of modification and elevation, has given origin to the highest group of living beings. With the promise before us of obtaining information regarding a most interesting group of animals, which are thus held by some _savants_ to possess relations of no ordinary kind to man himself, the reader will require little incentive to follow out the steps of a brief inquiry into their life-history and relations.