A REMARKABLE ORUSTACEAN PARASITE. 119 
wise inexplicable Cypris-larva point rather to a descent which 
may be thus expressed : 
Copepoda. Phyllopoda. Ostracoda. Ascothoracida, Cirrhipedia. 
ooo 
Synagoga. Laura. Petrarca. 
vi 
VA 
Cypridiform larva. 
Protostraca. 
The point of divergence between the phyla of the Ostracoda 
and Cirrhipedia would be characterised by a bivalvate organism 
with six pairs of thoracic appendages and some abdominal 
(Laura) segments, the reduction of which has proceeded further 
in the case of the Ostracoda than in the other groups (ef. 
Balfour, ‘Comp. Embryol.,’ i, p. 425). The existing Cypridi- 
form larva would approximately represent the (? hermaphrodite) 
ancestor of Cirrhipedia and of the Ascothoracidan type, while 
Petrarca may be regarded as having branched off from the main 
stem at a later period than its two congeners, when the deve- 
lopment of a terminal penis had been already acquired as a 
secondary character. 
To Canon A. M. Norman and to Professor E. Ray Lankester 
I desire to express my obligation for valuable suggestions. 
Notre.—The relation of Petrarca to its host is a case which 
may be termed either commensalism or parasitism. On the 
one hand, Tenia, an undoubted “ parasite,” does not derive 
sustenance directly from its host; on the other, many Amphi- 
poda, exhibiting no trace of degeneration, live in Anthozoa, 
and are generally ranked as commensals. 
