330 E. W. MACBEIDB. 



known Nauplius, which shows no signs of segmentation. "We 

 have however from comparative anatomy, strong reason for 

 believing that the ancestor of the Crustacea was segmented, 

 and that it was probably related to the Polychseta. How is 

 this apparent contradiction to be explained ? We answer that 

 the Nauplius retains the ancestral habits of Crustacea, and 

 with this a certain necessary amount of ancestral structure ; 

 but it has diminished in size and external segmentation has 

 disappeared. Since in the ancestor locomotion and prehension 

 of food were eflFected by one or two pairs of anterior append- 

 ages only, we have these alone represented in the Nauplius, 

 though the ancestor doubtless possessed in addition a series of 

 segments bearing undifferentiated parapodia-like appendages. 

 The complete disappearance of these is a mark of the high 

 specialisation of the larva; if we compare the various 

 families of the Entomostraca with one another, we find that 

 in the primitive group of the Branchipoda, the Nauplius shows 

 indications of a postoral segmentation; whereas in the highly 

 specialised Cirripedes and Ostracods we get a specialised 

 Nauplius. In the former case this is brought about by the 

 outgrowth of great spines, in the latter by the precocious 

 appearance of the adult bivalve shell, and in neither instance 

 is there a trace of segmentation. 



The larva of the Malacostraca the Zosea, has been a great 

 puzzle to morphologists. It is quite impossible to regard 

 some of the peculiar features, such as the suppression of the 

 thoracic segments and their appendages, as ancestral, and the 

 question has been raised by Claus,^ whether it has any 

 phylogenetic significance at all. 



Applying Sedgwick's principle, we explain the Zosea as 

 representing a later ancestral stage than the Nauplius, in 

 which some of the Nauplius appendages had become ex- 

 clusively masticatory and others exclusively tactile in function; 

 the main locomotor function had been, so to speak, passed on 

 to the two or three pairs of maxillipedes, which are 



^ L. Claus, " Zur Keunlniss d. Malakostrakenlarven," ' Wiirz. Naturw. 

 Zeitschrift,' 1861. 



