68 IOWA STUDIES IN NATURAL HISTORY 
cannot be called rudimentary in any true sense of the word, al- 
though they are so described by Geoffrey Smith in his treatment 
of the species.© On the contrary, they are well developed. The 
tip of the posterior gill seems exposed, projecting just back of the 
last thoracic segment and in front of the last and smallest pair of 
walking legs. The longest gills measure nearly an inch and a 
half and are altogether too well developed, it seems to me, for a 
rudimentary structure. Each gill is terete in form, tapering at 
each end and attached by its middle to the hard plates which limit 
the branchial cavity below. There are two series of leaf-like 
plates on each gill, borne on a hollow stem, the cavity of which 
is quite extensive in the preserved specimen; this hollow stem 
communicates with the body cavity at the point of attachment of 
each gill. It is not quite correct to call this a ‘‘hollow stem”’ be- 
cause the tube is formed by the union of two series of branchial 
lamelle near their distal ends; that is, the members of each pair 
of lamelle are thus joined and their extreme bases are also joined 
so that the ‘‘tube’’ is extraordinarily large, its greatest diameter 
extending from the extreme bases to near the distal ends of the 
lamelle. 
The branchial space is divided into a ventral chamber contain- 
ing the gills and a much larger dorsal chamber lined with branched 
villi. The gill side of the membranous partition is covered with 
hairs. Each gill plate is in reality a much flattened hollow sack, 
like a collapsed water bottle. On the under side of the hollow 
stem which bears the leaflets is a large number of hairs which 
under the microscope prove to greatly resemble the ‘‘cat-tails’’ we 
gather in the fall. Each has a very slender chitinous stalk, 
the distal part being covered with a dense mass of what looks like 
the fuzz or fur of the cat-tail. 
The membrane lining the entire upper, or pulmonary, chamber 
is thick and rubber-like and the inner surface resembles short moss 
owing to the numerous branched finger-like villi that project into 
the chamber. That part of the membrane immediately around the 
gills has simple villi that are surrounded by hairs which seem to 
radiate from the villi. Many of these hairs are somewhat hooked 
at their ends and interspersed among them are some of the ‘‘cat- 
tail’’ hairs already described. 
The part of the carapace forming the branchiostegite is ex- 
6 Cambridge Natural History, IV, p. 174. 
