22 PROCEEDINGS OF THE NATIONAL MUSEUM. 



VOL. XXVII. 



the larva of the first stage, is a free-swimming- form, resembling the 

 naupliiis stage of other Crustacea. Death occurred with all the larvte 

 of the first stage at the critical moment when the transformation into 

 the Cryptoniscian larva, or larva of the second stage was expected to 

 take place. Giard and Bonnier infer from this that it is highh^ prob- 

 able that under this form (the Cryptoniscian larva) the parasite 

 penetrates into the branchial cavity of its host, where it becomes 

 transformed into the adult. 



Sars has pointed out that there is an intermediate larval stage 

 between these two stages, and that this stage is the Microniscus stage. 

 He has shown that a true Microniscus develops from the first stage 

 of a normal Bopyrid larva, and that another form of Micronisciis after 

 having attained its normal development is transformed into the well- 

 known second larval stage or Cryptoniscian stage; that these two 

 forms of Microniscus, though similar in appearance, belong to two 

 difl'erent families of Epicaridea. Therefore he maintains that Micro- 

 niscus, which has been thought heretofore to represent a distinct genus, 

 the type of the family Microniscid», must in the future be regarded 

 as a transitory stage of development common to all Epicaridea. He 

 is inclined to think that this stage is always parasitic on Copepoda. 



The Cryptoniscian larvffi develop into adult males and females, the 

 females passing through a transitory stage of hermaphroditism. Tlie 

 larvsB of the first stage and of the Cryptoniscian stage have but six 

 pairs of legs, except in the Epicaridea, where the Cryptoniscian larvte 

 have seven pairs. The adult male in the Entoniscidre has but six pairs 

 of legs, due to retrogressive development. 



5. SIZE. 



The largest known Isopod is Batlnjnormus giganteus A. Milne 

 Edwards from the Gulf of Mexico. This form is 11 inches in length 

 and belongs to the family Bath3momidm. Other very large Isopods 

 are Chiridotea sahini (Kr0yer) and Chiridotea entomon (Linnseus). 



Perhaps the smallest Isopods are found among the Tanaid^e, the 

 Apseudidte, the Janirida3, and the Gnathiida\ Some of these forms 

 are only 2 mm. in length. 



Between these two extremes, 2 mm. and 11 inches, Isopods of 

 various intermediate sizes are known. 



6. HABITAT. 



According to their habitat the Isopods are classified as marine, fresh- 

 water, and terrestrial. 



The Oniscoidea are mostl}^ land forms, having air sacs developed in 

 their pleopoda, fitted for the respiration of air. As members of this 

 tribe IlajylophtJialmns puteus Hay, from an old well at Bloomington, 

 Indiana, Trichoniscus cavernicola Budde-Lund, from grottos in the 



