THOMPSON : A RARE THALASSINID. 
15 
and among those Thalassinidea in which the development has been 
studied the immature stages are not at all similar; viz., Axius (Cano, 
’ 91 ); Gebia (Cano, ’ 91 ; Sars, ’ 84 ) ; Callianassa (Cano, ’91 ; Claus, 
’ 76 ); Calocaris (Sars, ’ 84 ). 
The question then arises whether the possession of so unusual a 
common structure in the Calliaxis and Naushonia young is signifi¬ 
cant either as the result of a convergence perhaps due to some 
manner of life, or as indicating actual relationship between the 
parents. 
On the whole, ontogenetic stages among the Crustacea are very 
properly regarded as unsafe guides to relationship. Not only is 
there that strong tendency toward palingenetic structure through¬ 
out the class, which has given us the zoea and probably also the 
nauplius, but further, larvae belonging to closely allied species, as for 
example, those of Lucifer and Sergestes, or the young of brackish 
or fresh-water forms as compared with their marine congeners, are 
frequently very dissimilar in appearance. But it must not be for¬ 
gotten on the other hand, that the zoea and nauplius frequently 
serve as valuable criteria for relationship in the groups in which they 
occur, and the immature stages of species belonging to allied orders 
or families often possess common structural characters which may, 
moreover, be to a greater or less degree distinctive for the group 
or groups in question. The abdominal armature of Caridean zoeae, 
the shape of the eyes and telson in the majority of Thalassinoid 
and Paguroid larvae, and the peculiar zoea of the Brachyura are 
illustrations of this. A more striking instance still is the “phyllo- 
soma ” larva characteristic of the Loricata. 
Usually also when differences separate the young of related spe¬ 
cies, these are found on closer examination merely to mask a more or 
less fundamental likeness. But the peculiar structures which unite 
the young of Naushonia and Calliaxis, while separating them from 
the larvae of other Thalassinids, seem to be deep-seated. The 
asymmetry of the mandibles in the zoea and mysis stages is, so 
far as I can determine, unique among Malacostracan larvae at least. 
Typically, the zoea is symmetrical in all points, whatever the con¬ 
dition of the adult, and the final asymmetry enters during the later 
development. The ventral gland is also a very unusual structure. 
Claus ( ’ 84 ), indeed, hints at the presence of an homologous “Auf- 
treibung zwischen den Maxillen und vorderen Kieferfusspaar ” in 
