306 M. T. THOMPSON. 



these larvae are helpful in constructing a picture of the hypothet- 

 ical mating-stage phase of Penellcts metamorphosis. 



Attached to the host while possessing such a Baculns-\\kz 

 form, probably at first by the antennas, then later burrowing 

 into the tissues, the retrogressive metamorphosis begins. An 

 elongation of the posterior thoracic segment brings about a con- 

 dition in which the young Penella will parallel the earliest de- 

 scribed stage of P. various. Then the adult form is gradually 

 developed. The anterior part of the cephalothorax moulds itself 

 into the "head," the horns protrude from the posterior part of 

 this region, the thoracic segments coalesce, the anterior ones 

 remaining undeveloped, the posterior elongating to form the 

 major part of the body, while the abdomen develops the charac- 

 teristic lateral appendages. 



As regards the metamorphosis prior to the attainment of the 

 mating-stage form, we only know that the young Pcnellas almost 

 unquestionably hatch as nauplii. The analogies presented by 

 the development of Ltriuca and allied copepods (Scott, 'or, Claus, 

 '68) suggests the following cycle in the period between the nau- 

 plius and the completed, fertilized, mating-stage larva. The 

 nauplius probably metamorphoses to a cyclops-like form, the 

 so-called " Chalimiis" stage, and in this phase the young attach 

 themselves to some host. Typically among parasitic copepods, 

 the attachment in this chalimus stage in made by a frontal fila- 

 ment. While thus fixed on the host, the mating stage is reached 

 by successive moults. At this point in the ontogeny of Lerncea 

 the male and females copulate and the latter detach themselves 

 and for a short period return to a pelagic life. Then they settle 

 down upon the final host, which happens to be a different kind 

 of fish from the one they infested during the earlier part of 

 their life. Other parasitic copepods pass through a similar life 

 cycle on a single species of animal as host. Of course we have 

 no clue to which of these groups Penella belongs, but its 

 Lernaenoid affinities suggest a metamorphosis with the double 

 host. Nevertheless, this possibility should in no way discourage 

 the most careful scrutiny of freshly captured fishes that are 

 infested with Pendlas or are species which serve as hosts for this 

 genus, if opportunities present themselves. The metamorphosis 



