Class 1. Crustacea. 193 



veloped as tolerably powerful swimming organs^ tlie posterior 

 antennse and the mandibles (whicb last are wholly different 

 from their adult form) are each provided with a long setose 

 exopod. The larva possesses only the nauplius eye, the paired 

 (lateral) eyes are entirely wanting. It is actively free-swimming, 

 grows gradually in length; the other limbs arise, and after a series 

 of changes simultaneous with ecdyses, it finally attains the adult 

 form. Amongst other Crustacea, however, the young one hatches 

 at a more advanced stage, provided with several pairs of limbs, etc. 

 {see below) . 



Most Crustacea are marine; some creep about at the bottom of 

 the sea, others are excellent swimmers ; many are pelagic ; the 

 majority are free-swimming in the larval state. A few live in 

 fresh water, others on land or in damp places 



The Crustacea are divided into two sub-classes : Entomostraca 

 and Malacostraca. The latter group forms a circumscribed 

 whole, whilst the Entomostraca comprise several, and in some cases 

 only distantly related, groups. 



Sub-Class 1. Entomostraca. 

 Order 1. Phyllopoda. 



The head is furnished with a nauplius eye, and with com- 

 pound, stalked or sessile lateral eyes; the jaws are usually 

 feebly developed, sometimes also the antennee. The head is generally 

 followed by a thorax composed of numerous segments ; each thoracic- 

 somite bears a pair of strong, flat, leaf-like limbs, which 

 serve both as natatory organs and also as gills, and are about 

 equally developed on all the segments. The terminal joints of the 

 body are apodous (abdomen) ; on the most posterior is a pair of 

 jointed or unjointed backwardly-directed appendages. In the 

 majority of forms the body is entirely or partially covered with 

 a carapace which arises from the head. The Phyllopoda hatch 

 as nauplii. Most members of this small group live in fresh 

 water, as a rule in little pools. The eggs can endure complete 

 drought; some even do not develop until they have lain dry for 

 some time. 



1. Br-amcTiipMs possesses a pail- of stalked, movable lateral eyes: the 

 carapace. is wanting. The second antenna of the male is modified, to 

 hold the female during copulation. The thorax bears eleven pairs of legs, the 

 abdomen is nine-jointed, the caudal appendages unjointed. The organisms 

 belonging to this genns are transparent, elongate, and minute (1 — 2 c/m. in 

 length) ; f onnd in f resli-water pools ; they swim about constantly with the ventral 

 side upwards. The species of the nearly related genus Artemia {see page 67), 







