CRUSTACEA. 



403 



features concern the three posterior maxillipeds, which undergo atrophy, 

 and are either completely lost or reduced to mere u-njointed sacks (fig. 228). 

 In the stage where the complete Erichthus type has been reached, these 

 three appendages have again sprouted forth in their permanent form and 

 each of them, is provided with a gill-sack on its coxal joint. Behind them 



-== 



FIG. 228. ADVANCED ERICHTHUS LARVA OF SQUILLA WHEN THE THREE POSTERIOR 



MAXILLIPEDS HAVE BECOME REDUCED TO MINUTE POUCHES. (From ClaUS.) 



the three ambulatory appendages of the thorax have also appeared, first 

 as simple buds, which subsequently however become biramous. On their 

 development the full number of adult appendages is acquired. 



The most noteworthy points in the developmental history detailed 

 above are the following : 



(1) The thoracic and abdominal segments (apart from their appendages) 

 develop successively from before backwards. 



(2) The three last maxillipeds develop before the abdominal feet, as 

 biramous appendages, but subsequently completely atrophy, and then sprout 

 out again in their permanent form. 



(3) The abdominal feet develop in succession from before backwards, 

 and the whole series of them is fully formed before a trace of the 

 appendages of the three hindermost thoracic segments has appeared. It 

 may be mentioned as a point of some importance that the Zosea of Squilla 

 has an elongated many-chambered heart, and not the short compact heart 

 usually found in the Zosea. 



The younger stages of the Alima larva are not known 1 , but the earliest 

 stage observed is remarkable for presenting no trace of the three posterior 

 pairs of maxillipeds, or of the three following pairs of thoracic appendages. 

 The segments belonging to these appendages are however well developed. 

 The tail has its full complement of segments with the normal number of 

 well developed swimming feet. The larva represents in fact the stage of 

 the Erichthus larva when the three posterior pairs of maxillipeds have 

 undergone atrophy ; but it is probable that these appendages never become 

 developed in this form of larva. 



Apart from the above peculiarities the Alima form of larva closely 

 resembles the Erichthus form. 



Nebaliad.88. The development of Nebalia is abbreviated, but from 

 Metschnikoff's figures 2 may be seen to resemble closely that of Mysis. 

 The abdomen has comparatively little yolk, and is bent over the ventral 



1 The observations of Brooks (No. 493) render it probable that the Alima larva 

 leaves the egg iu a form not very dissimilar to the youngest known larva. 

 3 His paper is unfortunately in Eussian. 



262 



