THE CRUSTACEA OF NEW JERSEY. 41 



minute particles form the food in similar fashion to that of many 

 of the lower animals. Members of more than one order are 

 parasites, remarkable alike for their wide and often bizarre 

 deviation from: the typical structure of the class, and their 

 adaptation tO' their peculiar miode of life. In size they present 

 almiost every ^gradation from' microscopic water-fleas to crabs 

 two feet across the carapace, or four feet fromi tip to tip of legs. 



Geographically the distribution of all the chief groups are cos- 

 mopolitan, and only among the families, genera and species are 

 points of interest from this view: tO' be found. Fossils are known 

 from' very early time, though the oldest are usually referred to 

 the Phyllocarida, ranging from the Cambrian tO' the Trias. 

 Shells of Ostracoda are also known from the Cambrian upwards, 

 and those of Cirripedia from the Silurian. Arthrostraca are 

 known from palaeozoic times, but are rare as fossils. The earliest 

 Macruran is a shrimp-like form from the Devonian, while the 

 highly differentiated Birachyura are not known with absolute 

 -certainty until Cretaceous time. 



The recapitulation theory was first worked out in detail in the 

 Crustacea. Embryology shows that all Crustacea may be traced 

 back in individual development to the nauplius, upon which fol- 

 lows some kind of zosea-stage, many entomostraca progressing 

 no further. But in malacostraca the zosea is followed by the 

 mysis-stage, which is permlanent in schizopods and transient in 

 decapods. Though a tempting hypothesis it is only necessary 

 tO' recall that all such free larvae are subject to the action of the 

 struggle for existence, and may likely have been modified in 

 accordance with their own special needs and without reference 

 either to their ancestors or to the adult species into which they 

 finally change. 



Many Crustacea present instances of protective and aggressive 

 characters, as modifications in form;, color, etc., which serve to 

 conceal them from, their enemies or from their prey. Probably 

 the most striking example is that of certain crabs {Paramithrax) 

 which deliberately plant sea-weeds, sponges, alcyonarians, zo- 

 ophytes, etc., all over the carapace, and are thus perfectly con- 

 cealed except when in motion. Another crab (Dromia) carries 



