noies. 



CHAPTER IV. 



BROOKS'S Law. 



With a general description of tiie post-embryonal development of a few 

 species of Cypridiniformes and Halocypriformes. 



Among the material of the Stomatopods that was collected during the I nuoductonj 

 Challenger expedition, 1873—1876, there was also a very rich collection of pelagian 

 larvae. This caused W. K. BROOKS, who examined this material, to make an attempt to 

 „unravel the tangled thread of the larval history" of this group, one of the most difficult 

 problems that are presented in the study of post-embryonal development in the Crustacean 

 group. W. K. Brooks presented the result of these studies of his in a large work, „R e p o r t 

 on the Stomatopoda, etc.", 1886. 



In attempting to identify the different larval stages this author made use of (1) the 

 greater or less resemblance of the different individuals and (2) comparative measurements 



The method of comparative measurements seems to have given very good results, as 

 is shown by the following statement on p. 5: ,,the measurements usually enabled me to decide 

 with confidence whether a given larva does or does not belong to a certain series." From a 

 general point of view the greatest interest of the investigation is perhaps centred in this point. 



This method gave the best results in the study of four larvae that were caught 

 at the same time off Cape St. Vincent. This author writes as follows about this, p. 5: 

 ,,In a few cases these comparative measurements gave proofs of specific identity which 

 could hardly be made more conclusive by rearing the larvae. Thus the lengths of the series of 

 Coronis larvae shown in pi. XIII, figs. 1 — 8 are as follows, and if the length of the first stage 

 be successively multiplied by five-fourths of itself, and this number by five-fourths of itself 

 again, and so on, we obtain the series of numbers given in the second line, and as it is not conceiv- 

 able that an accidental collection of larvae should exhibit such exact conformity to a numerical 

 law, we may feel certain that these larvae are genetically related, that they belong to one 

 species or else to closely related species, and that the series is consecutive, with the exception 

 of one missing stage before the last. 



