48 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 95 



(e) similarly enters the posterior distal foramen of the bulb. The 

 second flagellum thus would appear to act as a plunger for driving 

 the sperm through the channel of the first. A strong transverse 

 muscle (B, i6p) crosses the middle of the bulb and evidently acts as 

 a compressor of the latter. 



The mating habits of the blue crab have been described by Hay 

 (1905) and by Churchill (1921). During the breeding season the 

 male crabs appear to be able to recognize females in a premoulting 

 condition. When a male encounters such a female he seizes her by 

 the back with his second, third, and fourth pereiopods, leaving the 

 chelae free for feeding, and the last pereiopods (swimming legs) free 

 for swimming. In this position the two crabs swim about for a day 

 or so ; but as soon as the female is about to moult, the male releases 

 her on the bottom of the water and stands guard until the shell is cast. 

 The female then turns ventral side up and extends the abdomen, 

 whereupon the male grasps her again, and insemination now takes 

 place while the female is in the soft condition following the moult. 

 The eggs are later fertilized as they are discharged through the semi- 

 nal receptacles, and are carried in a mass attached to the pleopods of 

 the female until they hatch the following spring. 



VIII. MYRIAPODA PROGONEATA 



The progoneate myriapods include the Symphyla, the Pauropoda. 

 and the Diplopoda. These forms evidently constitute a natural arthro- 

 pod group characterized by having the genital apertures in both sexes 

 situated on the third postcephalic segment, which segment most prob- 

 ably is the seventh postoral somite. To enumerate the postoral somites 

 in the progoneate myriapods, however, it is necessary to take a some- 

 what arbitrary view concerning the segmentation of the head and the 

 anterior body region, since the literature on the subject is full of 

 conflicting opinions and dififerent interpretations of the observed facts. 



If we start with the Symphyla as a basis for interpreting the 

 progoneate segmentation, the matter is relatively simple, for here the 

 gnathal region of the head supports a pair of mandibles and two dis- 

 tinct pairs of maxillary appendages, and the first postcephalic segment 

 carries a pair of legs. Though postantennal appendages (second 

 antennae) have not been discovered in any of the progoneate myria- 

 pods, Robinson (1907) reports the presence of a pair of "second 

 antennal " ganglia in the embryo of a diplopod. It is entirely reason- 

 able to assume, therefore, that the symphylid head contains four 

 postoral segments representing the second antennal, mandibular, and 

 two maxillary somites of the Crustacea, or the corresponding somites 



