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 



