24 Museum Notes

PIL ie, IEE

Fig. 1. Early “Zoea” larva of a “hermit” crab. (Genus Porcellus). X 20.
“2. Early “Zoea” larva of a true crab’ (Carcimnus moenas). X 14.
“3. “Megalops” larva of a “swimming” crab (Portunas holsatus). X 14.
“4, First larva of a “hermit” crab (Pagurus bernhardus). X 12.
“5. First larva of the common edible English “Grey shrimp.” (Crangon vulgaris).
Xe:
“6. A Schizopod or pelagic shrimp.

Parasitic copepod or fish louse (Lepeophtheirus pravipes Fraser) from the
Ling Cod (Ophiodon elongatus).

The larval stages cf some crabs and shrimps are shown in figures 1-5.

Most of these crustaceans carry their eggs until they hatch into “pre-zoea;” this
stage is quickly passed, then four or five “zoea” stages are entered upon, each stage
being marked by a moulting; after the next, called the “megalops” stage (Fig. 3), the
young differs only from their parents in size; they increase in size with each succeeding
moult until the maximum for th especies is reached.

The early larvae of crabs and shrimps are often difficult to distinguish (Figs. 4 and
5). The English common shrimp is hardly distinguishable, except by microscopic examin-
ation, from a B. C. shrimp Crangon nigricauda, and the zoea are probably indistinguish-
able from one another.

Known European forms are here drawn, as the local larval forms have not yet
been identified; to do this satisfactorily they should be reared from eggs to maturity.

With few exceptions, all crabs and shrimps are pelagic only when young; these
immature crustaceans are found in immense shoals and often at considerable depths;
they are a valuable fish feed.

A group of pelagic shrimps belonging to the two orders, Euphausiacea and Mysid-
acea, are entirely pelagic, and. owing to the great quantities in which they occur, are 2
very important constituent of the “plankton’—when they are visible fishermen call them
“red feed,” and the shoals may cover large areas. Salmon, herring and pilchards feed
greedily on them; they also form a food for several species of whales. Two species are
common here Euphausia pacifica and Thysanoessa spinifera; a Mysid Meomysis fran-
ciscorum (rayu?) is also found. Other species also no doubt occur: from San Diego
ten species have been recorded. They extend from the surface to considerable depths.
A microscopical examination is necessary’ to distinguish the species.

The parasitic copepod (Fig. 7) is very similar to L. salmonis, familiar to all users
of fresh salmon. When first hatched the parasitic copepods are nauplii similar to that
shown on pl. IIL, fig. 3, but nature always penalises a parisitic life, in this case by the
loss of limbs. They are not often found in thé “plankton.”