NO. 10 CRUSTACEAN METAMORPHOSES — SNODGRASS Jl 



There are two chief objections to this phyllopod theory of the origin 

 of jointed crustacean Hmbs. First, it gives no explanation of the origin 

 of the similarly jointed legs of other arthropods, except by the wholly 

 unsupported assumption that they likewise were developed from 

 phyllopodial limbs. Second, the ontogenetic development of the crusta- 

 cean appendages themselves gives no evidence of a phyllopodial origin, 

 and suggests, on the contrary, that the phyllopodium has been evolved 

 from an ambulatory leg. 



The study by Heath (1924) of the postembryonic development of 

 the branchiopod Branchiuecfa occidentalis shows very clearly that the 

 limbs arise as simple, lateroventral lobes of the body segments (fig. 

 3 B). Instead of taking on a phyllopodial shape, the rudiments grow 

 out first in a slender leglike form (C, D). On the inner margins of 

 the appendages at this stage there are indentations suggestive of an 

 incipient segmentation, and at the apex is a terminal lobe. Only at a 

 later stage (E) do the appendages become broad overlapping flaps. 

 Finally in the adult (F) the appendages have taken on the form of 

 typical unsegmented phyllopodia with three large flat exites, six 

 endites, and a free, independently musculated terminal lobe. Clearly, 

 these appendages in their ontogenetic development undergo a meta- 

 morphosis from an ambulatory leg into a phyllopodium. Though 

 Heath himself did not have this phase of the subject in mind, his 

 pictures speak for themselves. 



Conversely, as seen in Fleegaard's (1953) account of the post- 

 embryonic stages of the decapod crustacean Pcnaeus set if ems, the 

 rudiments of the pereiopods develop directly into legs without under- 

 going any stage suggestive of a phyllopodial origin. The pereiopods 

 appear during the second protozoeal stage as simple lobes on their re- 

 spective body segments (fig. 28 A). In the third protozoea they take 

 on a biramous structure (B), in which the protopodite, at first un- 

 divided, bears a short unsegmented endopodite and a longer exopodite. 

 In the second my sis stage (C) the limbs attain a fully segmented struc- 

 ture by the division of the protopodite into two segments and the 

 endopodite into five, with a terminal chela on each of the first three. 

 The exopodites are now large seta-bearing branches of the basipodites 

 used for swimming. In the postmysis (D) the pereiopods have become 

 essentially uniramous by the reduction of the exopodites to small lobes, 

 and the swimming function has been taken over by the pleopods. This 

 condition is retained in the adult. If the pereiopods of Penaeus had a 

 phyllopodial origin in their phylogeny, there is nothing to suggest it in 

 their ontogeny. The mouth-part appendages proceed along their own 

 lines of development to serve the special functions they have assumed 



