Dr. F. Miiller on the Development of the Stomapoda. 13 II. — On the Developmental History of the Stomapoda. By Dr. Fritz Muller, of Desterro*. [Plate II.] Under the name of Zoea we have long known young states of the Crabs and Hermit-Crabs, distinguished especially by the want of the ten feet to which the adult animals are indebted for their name of Decapoda. I have recently described the Zo'ea-forms of the PorcellancE as those approaching most closely to those of the Crabs. But in certain Prawns and Stomapoda, as I have since ascertained, similar conditions occur. Of the meta-morphosis of the former, which commences sometimes (as in the Cirripedia and Rhizocephala) with Monoculoid forms and passes through very peculiar Zoeoid and Mysis-\\Ve states, and sometimes with Zoea-forms which in structure and mode of movement resemble those of the Hermit-Crabs, whilst in others we can hardly say that there is any metamorphosis, I hope shortly to be able to give a tolerably complete account. In the case of the latter I have at present no prospect of fresh observa-tions, and therefore communicate what I have recorded upon the only larva yet discovered. The little animal (PI. II. fig. 1), which is 3'25 mill, in length, has the general form and likewise all the glassy transparency of an Alima. The segments exist in almost the same number as in mature Stomapods, only the sixth and seventh abdominal segments being not yet distinct from each other ; as in the Zoe(B of the Cx-abs and Porcellance, the appendages of the six hinder thoracic segmentsf and the lateral laminae of the caudal fiin J are still entirely deficient. * Translated from Wiegmann's Archiv, 1862, by W. S. Dallas, F.L.S. f I have never been able to reconcile myself to the exceedingly forced notion which limits the thorax of the Crustacea, like that of Insects, to three segments. It is contradicted, as appears to me, by the develop-mental history of those Crustacea which are subject to a metamorphosis, whilst the ordinary and readily perceptible division between the thorax and abdomen is confirmed thereby. It is only a reference to the Insecta that could have led from this and to that new artificial line of demarcation. But if any Crustacea can be compared with certainty (as regards the divi-sions of the body) with Insects, these are certain Zoea-forms {e.g. of Pa-gurus) with three pairs of buccal organs, three pairs of legs, and an abdo-men destitute of appendages. These three pairs of feet certainly become the foot-jaws of the Crab in accordance with the notion referred to; but the five pairs of true feet of the Crab are produced, not from the abdomen of the Zoea whilst a new " postabdomen " sprouts forth behind, but they are formed in front of the abdomen, and often simultaneously with and in the same form as the third pair of foot-jaws. They are to be regarded as an addition to the thorax which is entirely wanting in Insects ; and here, again, the process is repeated, that after the appearance of new posterior legs the antei'ior ones give up their original function, and become feelers or manducatory organs. X The distinction of the last two abdominal segments, which usually
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