CRUSTACEA. 



401 



FIG. 225. MEGALOPA STAGE OF CRAB LARVA. 



form. In other cases again (e.g. Portunus) the Zosea spines are completely 

 thrown off at the youngest Megalopa stage. 



There is a gradual 

 passage from the young- 

 est Megalopa to the adult 

 form by a series of moults. 



Some of the brachyurous 

 Zoaaa forms exhibit consider- 

 able divergences from the 

 described type, more especi- 

 ally in the armature of the 

 shield. In some forms the 

 spines are altogether absent, 

 e.g. Maja (Couch, No. 474) 

 and Eurynome. In other 

 forms the frontal spine may 

 be much reduced or absent 

 (Inachus and Achaeus). The 

 dorsal spine may also be ab- 

 sent, and in one form de- 

 scribed by Dohrn (No. 478) 

 there is a long frontal spine 

 and two pairs of lateral 

 spines, but no dorsal spine. 



Both dorsal and frontal spines may attain enormous dimensions and be 

 swollen at their extremities (Dohrn). A form has been described by Glaus 

 as Pterocaris in jvhich the cephalo-thoracic shield is laterally expanded iuto 

 two wing-like processes. 



The Zoaea of Porcellana presents on the whole the most remarkable 

 peculiarities and, as might be anticipated from the systematic position of 

 the adult, is in some respects intermediate between the maci'urous Zosea 

 and that of the Brachyura. It is characterized by the oval form of the 

 body, and by the presence of one enormously long frontal spine and two 

 posterior spines. The usual dorsal spine is absent. The tail plate is rounded 

 and has the character of the tail of a macrurous Zosea, but in the young 

 Zosea the third pair of maxillipeds is absent and the appendages generally 

 have a brachyurous character. A Megalopa stage is hardly represented, 

 since the adult may almost be regarded as a permanent Megalopa. 



Stomatopoda. The history of the larval forms of the Stomatopoda 

 (Squilla etc.) has not unfortunately been thoroughly worked out, but 

 what is known from the researches of Fritz Miiller (No. 495) and Claus 

 (No. 494) is of very great importance. There are it appears two types, 

 both of which used to be described as adult forms under the respective 

 names Erichthus and Alima. 



The youngest known Erichthus form is about two millimetres in length, 

 and has the characters of a modified Zoaaa (fig. 226). The body is divided 

 into three regions, an anterior unsegmented region to which are attached 

 two pairs of antennae, mandibles, and maxillae (two pairs). This portion 

 bas a dorsal shield covering the next or middle region, which consists of 

 five segments each with a pair of biramous appendages. These appendages 



B. E. 



26 



