FISHERY BULLETIN: VOL. 84, NO. 2 



lustrated its maxilliped (their fig. 6b, c). Gooding 

 (1963) described features of Copepodid I of Leptino- 

 gaster major. 



Before her death Mildred S. Wilson had studied 

 specimens of Leptinogaster (= Myocheres) major 

 that had been sent to her from Rhode Island and 

 Prince Edward Island, and had prepared the first 

 draft of a redescription. She recognized the need for 

 a thorough redescription of this species whose 

 original description by Williams (1907) is very incom- 

 plete Although she wrote (1950) that a detailed 

 description of adults and developmental forms was 

 then in preparation, this study apparently was never 

 completed. In a letter to J. C. Medcof dated 24 

 August 1948 she stated that she had found two early 

 stages of Myocheres. Presumably descriptions of 

 these copepodids would have been part of her pro- 

 jected study if she had lived. 



During the study by Humes and Cressey (1960) a 

 large number of Leptinogaster major (1,535) were 

 collected from Mya arenaria over a period of almost 

 2 yr at Cotuit, MA. The copepodids and adults 

 described below came from collections made during 

 the summer of 1957. All five copepodid stages, 

 distinguished on the basis of the number of body 

 segments, as well as adults, were obtained. This 

 paper deals with the detailed description of the ex- 

 ternal morphology of these immature stages and 

 adults. 



Although the copepodids described here were not 

 obtained by rearing, it seems certain that the cope- 

 podids found in such large numbers are those of Lep- 

 tinogaster major. No other species of copepods oc- 

 curred in the Mya arenaria examined. 



MATERIALS AND METHODS 



The copepodids and adults described here were 

 selected from a pool of 305 copepodids and 195 

 adults found in 125 Mya arenaria during May- 

 September at Cotuit, MA. The successive Cope- 

 podids I-V and the adults were cleared in lactic acid 

 and sorted by size and external morphology into 

 their respective groups. 



All measurements and dissections were made on 

 specimens cleared in lactic acid, following the 

 method of Humes and Gooding (1964). The body 

 length does not include the setae on the caudal rami. 

 The measurements of certain parts, such as the 

 length of the first antenna, maxilliped, and various 

 setae and claws, and the dimensions of leg 5, the 

 caudal ramus, and the urosomal segments, are based 

 on dissected specimens from which the drawings 

 were made, and may be considered representative 



of nearly average body size Such measurements are 

 intended more to show relative changes in size dur- 

 ing successive instars rather than to represent ab- 

 solute size The drawings were made with the aid 

 of a camera lucida. The abbreviations used are as 

 follows: A l = first antenna, A 2 = second antenna, 

 L = labrum, MD = mandible, MX X = first maxilla, 

 MX 2 = second maxilla, P 3 = leg 3, P 4 = leg 4, and 

 P 5 = leg 5. 



DESCRIPTIONS 



Copepodid I 



Figures la-n, 2a-c 



Size— Length 0.57 mm (0.45-0.60 mm) and 

 greatest width 0.17 mm (0.16-0.18 mm) based on 38 

 specimens. 



Body form (Fig. la, b, c).— Saphirella-\\ke, with 

 cephalosome bluntly pointed anteriorly. Five body 

 segments including and posterior to segment bear- 

 ing leg 1. Anal segment with 4 groups of spines, 2 

 ventral groups and 2 ventrolateral groups (Fig. 

 Id). 



Caudal ramus (Fig. le).— Relatively short, 36 x 18 

 /urn, ratio 2:1, with 6 setae. Outer lateral seta 18 ^m, 

 dorsal seta 20 ^m, 4 terminal setae from outer to 

 inner 23, 17, 39, and 176 pm, the last with minute 

 lateral spinules. 



Rostrum (Fig. lf).—Broad ridge, prominent in 

 lateral view (Fig. lg). 



First antenna (Fig. lh).— Five-segmented, 83 pm 

 long. Armature: 2, 2, 3 + 1 aesthete 2 + 1 aesthete 

 and 5 + 1 aesthete All setae smooth. 



Second antenna (Fig. li).— Indistinctly 4-segment- 

 ed, last segment obscure First segment with 1 distal 

 seta. Second segment with 1 seta and group of small 

 spines. Third segment with outer row of spines and 

 2 slender inner setules, with outer stout curved seta 

 having expanded serrate distal half and 1 short in- 

 ner blunt seta. Fourth segment small and indistinctly 

 set off from third segment, with 1 blunt short seta, 

 1 long stout smooth seta, 1 slender smooth seta, and 

 1 long stout seta with prominent lateral setules. 



Labrum (Fig. lj).— Broad, with ventral surface 

 bearing 2 medially interrupted rows of spines and 

 with posteroventral margin having row of small 



228 



