22 



The Canadian Field-Naturalist. 



[Vol. XXV. 



ably suited for propelling the animal, mov- 

 ing consecutively, as grain stalks before the 

 wind, but they have also respiratory im- 

 portance, being subdivided into inner and 

 outer parts, and even those far from the 

 mouth have "gnatho-bases" (chewing 

 parts), and pass along by their movements 

 any food that is secured. The last pair is 

 modified for reproductive purposes. The 

 conspicuous genital organs mark the bound- 

 ary between the prae- and the post- 

 genital regions, both mostly of about the 

 same length. With the ripe males the 

 copulatory organ presents a mostly bifid, 

 smaller bag; while with the females it is 

 an oblong or more rounded (elliptical) 

 sack containing, at the right time of the 

 year, the eggs. The tail is mostly long and 

 slender and consists of 8-9 joints; it ends 

 in two furcal rami (cercopods), only ex- 

 ceptionally united {Thamnocephalus). 



Except when strongly colored the ani- 

 mals are so transparent that the internal 

 parts are to be seen plainly with a mag- 

 nifying glass; most conspicuous is the long 

 slender "heart" extending through nearly 

 all the trunk-segments (somites) dorsally 

 and with a pair of openings (ostia) on 

 each of these. The alimentary canal is 

 also conspicuous, owing to the food filling 

 it out ; it is seen stretching as an almost 

 uniform tube to the end of the last tail 

 segment, where it opens. The maxillary 

 gland (excretion organ) is also conspi- 

 cuous on the underside of the head (it is 

 especially large in the younger stages) ; and 

 in the males the white testes and their auxil- 

 iary organs (vasa deferentia) are plainly 

 seen in living individuals, though not so 

 conspicuous as the female's unripe (ovar- 

 ial) or ripe eggs. Less conspicuous are 

 the ladder-like nervous sj'stem (though the 

 brain is large and well defined) and the 

 little differentiated blood vessels. 



The fairy-shrimps have this in common 

 with most of the Crustacea, that their 

 young stages are quite unlike the full- 

 grown animals. The egg hatches into a 

 larva, the so-called nauplius or metanau- 

 plius. It is often only the size of a pin- 

 head, but usually of a vivid, red color, and 

 of an oval or pear-shape. The first pair 

 of antenna is longer than in the full- 

 grown individuals and pointed directly 

 ahead ; they are probably used as balancing 



more than as locomotory organs. Between 

 them is the large median e.ye. There fol- 

 lows the second pair of antenn* greatly 

 developed as the principal swimming or- 

 gans and divided up into several long, 

 spined branches. Of the mouth parts the 

 mandibular-palps are the most developed 

 and leg-like ; they also help in swimming, 

 having long hairs. The somites carrying 

 the foliaceous legs are little differentiated, 

 and the more posterior ones as yet only 

 represented by hairy serrations on the 

 under side of the "abdomen". No tail is 

 yet present, the hind-end of the larva being 

 rounded-tapering. These nauplii make up 

 for their inconspicuous size by their vio- 

 lent movements; they probably subsist 

 right after hatching for a time on the yolk 

 they contain. 



The nauplius groAvs rapidly both in 

 length and in the development of the ap- 

 pendages and soon reaches the metanau- 

 plius stage, which is so-termed owing to the 

 presence of larval and adult characters. 

 Thus the second pair of antennie and the 

 mandibular palps are still large and the 

 principal swimming organs, and of the 

 foliaceous legs and abdominal segments 

 only the foremost are developed to any ex- 

 tent and well marked off from the suc- 

 ceeding ones The tail is short and clumsy 

 and little differentiated from the abdomen, 

 merely tapering from the latter. On the 

 other hand there are now besides the me- 

 dian nauplius eye two large composite 

 eyes, though their peduncles are less pro- 

 nounced than in the fullgrown individuals ; 

 and the maxillary gland seems to reach its 

 highest development (size) in the meta- 

 nauplius stage. The more oblong shape of 

 the whole body, the beginning differentia- 

 tion and development of somites and foli- 

 aceous legs and tail, and the proportions 

 of the various appendages compared with 

 the length of the whole animal, also make 

 this stage very distinct from that of the 

 nauplius. 



Gradually the metanauplii grow in 

 length and take on the appearance of the 

 adults. The foliaceous legs all become 

 fully developed and assume their loco- 

 motory duties, and simultaneously the 

 second pair of antennae becomes more rudi- 

 mentary (females) or transformed into the 

 claspers (plus accessories) of the males. 



