248 



CRUSTACEA. 



we find, in the first group, a more gradual transition through 

 many moults from the NaupUus to the adult form, while in the 

 Malacostraca the metamorphosis lias attained a higher degree of 

 specialisation, inasmuch as the separate stages appear more distinct 

 from one another, and larval stages are intercalated which are not 

 on the direct path of transition from the young to the adult form. 

 These latter, by the development of secondary peculiarities, attain 

 a certain independence and lead on to the adult condition only 

 through further important changes in form. The metamorphosis of 

 the lower Crustacea thus bears the same relationship to that of the 

 Malacostraca as does incomplete to complete metamorphosis amono- 

 the Insecta. As examples of these newly-introduced stages in the 

 ontogenetic process, we must mention especially the Zoaea of the 

 Decapoda and the Zoaea-like stage of the Schizopoda {CaJyptopis) and 

 the Stomatopoda, which are distinguished by the fact that although 

 they possess the full nu)nber of body-segments, those of the middle 

 region of the body are in a rudimentary condition. In this larva 

 the posterior (five to seven) thoracic segments seem to be unusually 

 backward in developing, and to have either no limbs or only very 

 rudimentary ones, while the segments of the abdomen are already 

 highly developed. The Zoaea is evidently a larval form secondarily 

 changed by marked adaptation to a pelagic manner of life. From 

 this standpoint it seems appropriate enough that the body retains 

 a compressed form, tliat the most important locomotory organs (the 

 maxillipedes and in some cases the antennae) are developed in the 

 anterior region of the body, and that a posterior movable region 

 (abdomen) develops early for swimming and steering purposes. The 

 rudimentary condition of the middle region of the body thus ap- 

 pears to some extent explicable. 



In regarding the Zoaea as a larval form secondarily intercalated in tlie course 

 of develojjment, which has attained a certain independent value and significance, 

 special interest attaches to the development of the heart. Bearing in mind the 

 condition of the heart in Ncbalia and the Schizopoda, we should expect to find, 

 in the larval forms of the Decapoda, a long tubular heart. We should also 

 presuppose tliat the three pairs of ostia occurring in the Decapodan heart would 

 already be found in the heart of the Zoaea. This, however, is not the case. 

 The lieart of the Zoaea is a short sac, recalling to some extent that of the 

 Copepoda. It has only two pairs of lateral ostia (in individual cases, such as 

 Fenacus, Euphausia, only one). The missing pairs of ostia only appear later. 

 This proves clearly that the heart has undergone secondary modification corre- 

 sponding to the requirements of the Zoaea. The phylogenetic stages in the 

 development of the heart have been modified to suit the altered conditions of 

 organisation of the Zoaea stage. 



