410 COMPARATIVE ANATOMY CHAP. 



characteristic biramose thoracic feet does not once occur. The larval history of the 

 Euphausidce, Stomatopoda, and Carididce, the early developmental processes in the 

 Loricata which take place within the egg, and the so-called direct development of so 

 many Malacostraca show that the Malacostracan body becomes gradually differen- 

 tiated in the typical manner (as in the Entomostraca] from before backward. The 

 larval history of Penaeus shows further that, with the exception of the 6th pair of 

 pleopoda, the extremities also begin to form and differentiate from before backward. 

 In the Euphausidce the extremities of the posterior thoracic region differentiate 

 almost simultaneously with those of the abdomen from before backward. 



It is of the greatest importance for the comprehension of the typical Decapodan 

 Zocea, and especially of that of the Brachyura, to bear in mind the established fact 

 that in that larva, in spite of the apparent absence of the thoracic region comprising 

 the 4th to 8th thoracic segments, the thoracic ganglionic mass through which the 

 sternal artery runs has already begun to form. 



Not less important also is the fact that the Stomatopodan larva possess an 

 elongated dorsal vessel with many pairs of ostia, while the Decapodan Zocea and the 

 other Thoracostracan larvae which resemble it possess a compact thoracic heart, 

 generally with 1 or 2 pairs of ostia. These facts sufficiently show how varied are the 

 Thoracostracan larvae which have been artifically combined into a Zocea group. 



In all future attempts to understand the morphology of Crustacean larval forms 

 the following considerations will probably have to be taken into account. 



The grade of development and physiological importance of a section of the body 

 or of a pair of limbs in the adult animal may be recognised by the earlier or later 

 appearance of their rudiments ; e.g. the extraordinarily early' appearance of the last 

 pair of pleopoda in the Thoracostraca, i.e. of that pair of pleopoda which in the 

 adult differs so strikingly from the others, forming together with the telson the 

 caudal fin, which is the chief organ of locomotion. A contrast to this is afforded by 

 the late appearance of the reduced maxillae of the adult Phyllopoda and Cirripedia. 



Not only the special form of the adult animal, however, but also that of one or 

 more of its larval stages may have some influence on the early processes of develop- 

 ment, i.e. when these larval stages are not merely phases of development, but 

 animals feeding themselves and leading an independent life and playing an important 

 part in the life economy of the species, more or less like adult animals though unable 

 to reproduce themselves. Such larvae show throughout their organisation independ- 

 ent adaptation to the special conditions of their existence, and it is the first and 

 chief object of the preceding developmental stages to prepare their organisation, 

 not that of the adult animal. From this point of view we shall perhaps some day 

 be able to explain the Brachyuran Zocea with its reduced thorax and failing 4th 

 to 8th ambulatory feet, which in the adult animal are the only organs of locomotion, 

 but in the Zocea are perhaps useless or even a hindrance. The larvae lead a marine 

 life, and their organisation is no doubt more or less adapted to this life. "We see, 

 therefore, how important it is to ascertain what are the functions of the different 

 parts of each larva, in order to arrive at any sure conclusion as to the significance of 

 the changes it undergoes. 



One of the most important and most interesting problems of ontogeny is the 

 reduction and subsequent reappearance of the same portions of the body, of 

 which the larval history of the Malacostraca yields us so many examples. (Compare 

 especially the larval history of the Stomatopoda and Loricata. ) For the solution of 

 this problem also the above points of view may prove of service. In'the Stomatopoda 

 the last 3 pairs of thoracic feet, the so-called \ ambulatory feet, like the 5 pairs of 

 ambulatory feet in the Brachyura, first appear at the end of larval life. The 5 

 anterior pairs, however, develop during the first period of larval life (youngest 



