ARTHROSTRACA AND CUMACEA. 



149 



stage of development, the embryos consequently showing throughout 

 embryonic life a ventral curvature of the abdomen (Fig. 75). 



Although, as a rule, the limbs appear to take their rise almost simultaneously, 

 yet in Asellus, whose ontogeny exhibits several primitive features, we find a 

 NaiipUus stage* characterised by the presence of two pairs of antennae and 

 the secretion of a larval ciiticle (Fig. 74 A). The mouth-parts and six pair 

 of thoracic limbs, and, finally, the abdominal appendages appear later (Fig. 



..to 



Fig. 74.— Two stages of development o. Asellus, seen from the side (diagrammatic). A, 

 NaupHus stage (Van Beneden). B, older stage (Dohrn). a', first antenna; o", second 

 antenna; af, anus; I, lobe-like appendages; md, mandible; mx', first maxilla ; m.r", second 

 maxilla ; mf, maxillipede ; I-VI, first six pairs of ambulatory limbs ; 1-5, first five pairs of 

 pleopoda ; m, mouth ; x, vitelline membrane ; y, blastodermic cuticle. 



74 B). After the rudiments of the limbs have appeared, paired prominences 

 form behind the mouth ; these become the rudiments of a bilobate lower lip 

 {pa7r(g7iatha). 



The course of development of the germ-band observed in Asellus is connected 

 with that in Ligia by the presence of a distinctly recognisable Nauplius stage 

 (Fig. 69, p. 138). Tlie chief point worthy of 

 notice in the later development of this form is 

 that the rudiments of the thoracic limbs are 

 originally biramose (Nusbaitm, No. 85a), and 

 that the adult limbs arise from these by the 

 suppression of the exopodite. This is in har- 

 mony with the presence of vestigial exopodites 

 on the two anterior pairs of thoracic limbs 

 of the Anisopoda, and supports the deduction 

 that the Isopoda, and with them of all Arthro- 

 straca, arose from ancestors resembling the 

 Schizopoda. 



A feature apparently universal in the Isopoda 

 is the temporary suppression in the embryo of 

 the posterior of the seven pairs of ambulatoiy 

 limbs (thoracic limbs), and its re-appearance in the young only after leaving 



* Former observers failed to find a rudiment of the mandible in the Nauplius 

 stage of Asellus, but Boas (No. 4, Crustacean I\Ietamorphosis) was able to prove 

 the presence of such a rudiment in a few individuals. 



Fig. 75.— Embryo of an Amphi- 

 pod (Corophium, F. Mulleb). 

 A', dorsal organ. 



