CARIDEA. 271 



as well as the five following thoracic limbs (IV-VIII) are still large 

 biramose limbs with well-developed exopodites. 



In the transition from the My sis stage to the shrimp form, the 

 exopodites of the thoracic limbs are reduced, while the flagella of 

 the antennae become jointed, and the pleopoda which appear in 

 the Mysis stage attain their full development. The second and 

 third maxillipedes retain the character of biramose limbs. The 

 second maxillipede has a geniculate endopodite, a smaller exopodite 

 (flagellate appendage), and gills ; the third maxillipede has its 

 endopodite developed as a long ambulatory limb, while, in it also, 

 the exopodite persists as a flagellate appendage. The three following 

 limbs (first, second, and third ambulatory limbs) are provided with 

 rudimentary pincers as early as the Mysis stage. 



The various ontogenetic stages of Penaeus, which have become known through 

 the examination of the Challenger material, can easily he brought into agreement 

 with the process of development just described. They, however, show many 

 variations which may be regarded as generic and specific differences ; these 

 chiefly affect the armature of spines (especially in the abdomen) and the relative 

 development of the limbs (the thoracic limbs and the pleopoda). 



The remarkable Peteinura gubernata, which SPENCB Bate (No. 100), on 

 account of certain points of agreement with Aristeus, regarded as possibly 

 belonging to the Penaeidae, must no doubt also be considered as a larval form. 

 This form agrees closely with that described by Dohrn (No. 121) as Ceratapsis 

 longiremis, in the armature of the cephalo-thorax, the shape of the limbs, and 

 the excessive development of the exopodite of the sixth pleopod. Ceratapsis 

 is regarded by Boas and Clatjs as a larva of some form which must be related 

 to the Penaeidae on account of the structure of its gills, made known by the 

 latter author. 



The metamorphosis of Stenopus, which is allied to Penaeus and shows some 

 relation to that of the Sergestidae, has been described by Brooks and Herrick 

 (No. 111). The larva which hatches from the egg is a Protozoaea with sessile 

 eyes, antennae serving as swimming limbs, a deeply-cleft telson, and a long ros- 

 trum, and with all the pairs of limbs as far back as the first thoracic appendage 

 (inclusive) already developed. The posterior region of the body is only indis- 

 tinctly segmented, and has no appendages. From this form a true Zoaea 

 develops, showing the characters of a Caridid Zoaea (p. 272). A later stage 

 is characterised by the enormous lengthening of the fifth pair of ambulatory 

 limbs, which function as the principal locomotory organ of the larva and equal 

 the whole body in length. These limbs are reduced in the following Mastigopus 

 stage to small vestiges, and the preceding pair (fourth ambulatory limbs) also 

 degenerate for the time only. In this respect, Stenopus recalls the Sergestidae 

 (p. 266). 



C. Caridea. 



The metamorphosis of the Caridea (Palaemonidae, Alpheidae, 

 Crangonidae, etc.) is, as compared with that of Penaeus, essentially 

 abbreviated. The Nauplius and Protozoaea are never free stages 



