FREE-SWIMMING COPEPODS OF THE WOODS HOLE REGION. 
183 
well-developed, with long bristles. Posterior maxilliped 4-joiuted. Inner ramus of tirst pair of feet 
2-jointed; fifth pair of feet of female without inner ramus, with a long outer bristle on second basal 
joint. 
Female: Cephalothorax with 5 segments ; the two last thoracic segments fused ; the last sometimes 
terminating in lateral joints. Rostral filaments delicate or wanting. Abdomen with 3 segments, 
symmetrical. Anterior antennae 17-joiuted; joints 2 to 4, 5 and 6, 7 and 8, 9 and 10, 12 and 13, 24 and 
• 25 fused; aesthetasks delicate, filamentous. Posterior antennae very slender, their distal basal joint 
fused with proximal joint of inner ramus; inner ramus with much elongated terminal joint; the very 
short outer ramus with relatively long proximal (first and second joints fused) and very short distal 
joints. Mandible with feeble, 7-tootlied blade. Maxilla with abortive rami, represented only by long 
bristles, but with well-developed lobes. Anterior maxilliped with long curved bristles even on proxi- 
mal lobes. Posterior maxilliped 
similar to that of Parapontella, 
but without outer marginal bris- 
tles and with three inner marginal 
bristles on third joint. First to 
fourth pair of feet with 3-jointed 
outer and 2-jointed inner rami; 
first basal joint without a bristle, 
second basal joint with a rather 
long outer marginal bristle on 
fourth pair; bristles of rami as 
follows : Outer ramus of first pair 
with 1, 1, 2 outer and 1, 1, 4 inner 
marginal bristles ; second to fourth 
pairs with 0, 0, 0 outer and 1, 1, 5 
inner marginal bristles; inner ra- 
mus of first pair with 0, 1 outer 
and 1, 5 inner marginal bristles; 
second and third pairs with 0, 1 
outer and 2, 6 inner marginal bris- 
tles, fourth pair with 0, 1 outer 
and 3, 5 inner marginal bristles. 
The much reduced fifth pair of feet consists on either side of 2 or 3 joints; end joint a thick, stylet- 
shaped bristle, and on outer margin of second basal is a delicate plumose bristle. 
Male: Sexual differences in structure of trunk, anterior antenme, and fifth pair of feet. Peculiari- 
ties in articulation of thoracic segments and sometimes in form. Abdomen with 5 segments; genital 
orifice sinistral ; fourth segment and furca abbreviated. In both anterior antennae the proximal joints 
are incrassated, and the ninth is separated from tenth, the twelfth from thirteenth joint; spines want- 
ing; right anterior antenna transformed into a grasping organ, and joints 19 to 21, and 22 to 25 are 
fused. Fifth pair of feet consisting of a common median piece, a right 4-jointed, and a left 3-jointed 
uniramous foot; especially the right is claw-shaped, and forms an incomplete forceps, with a process 
on penultimate joint. 
20. Acartia tonsa Dana; a, female, dorsal aspect; 6, male, dorsal aspect; c, male, 
fifth pair of feet; d, female, fifth pair of feet. 
20. Acartia tonsa Dana. 
Rostral filaments present; last thoracic segment rounded on sides. Abdomen relatively short, 
without thorns; anal segment hairy on either side; in male with fine pointson second segment. Ante- 
rior antenme of female not reaching posterior border of genital segment. For fifth pair of feet, see fig. 
20, c and d. 
Colorless and translucent. 
Length of female, 1.3 to 1.5 mm. ; of male, 1.05 mm. 
This species was found to be one of the commonest copepods in tow taken from wharf of Fish 
Commission at Woods Hole during July and August, 1899. It appeared abundantly also in tow from 
Plymouth Harbor, but was very scarce in Gulf Stream 70 miles south of Marthas Vineyard. Hitherto 
the species has been taken only at Port Jackson, Australia (Dana), and west of South America, between 
Valparaiso and Callao (Giesbrecht). Acartia clatisii Giesbreclit, which one would expect to find in 
our waters, could not be recognized in the samples of the tow. 
