PYCNOGOXIDA. 15 



large and small yolk parts, the maeromeres and micromeres of Morgan, must be of a considerable 

 influence for the later development, and that it is connected with or proportioned to the mass of the 

 alimentary volk in the egg; this fact again plays a very great part in the biology of the larva, as 

 this latter may exist without any other food, and keep enclosed in the safe egg-shell the longer, the 

 more alimentary yolk it brings along with it. Kroyer already has referred to this reciprocal relation 

 in his Contributions to the knowledge of the Pycnogonida* (1845), comp. especially the third of the 

 five principal heads, under which he collects the results of his examinations, I.e. p. 137, and to which 

 I have referred at p. 12 seq. 



A germinal stripe as in the other Arthropoda, especially the Insecta, is not 

 formed. The ganglia as well as the separate pairs of limbs are formed or constricted 

 by degrees, from before backward, and, with the exception of the three foremost 

 ganglia and pairs of limbs, one after the other. 



In the Arthropoda, especially the Insecta, the first germ of the embryo, as is well known, is 

 distinctly seen as a smaller or broader longitudinal band, the germinal stripe, along the under side 

 of the egg, and from this band the formation of the abdominal nerve cord and the pairs of limbs take 

 their rise almost at the same time; besides the formations are only small, and, as far as they really 

 are developed, they grow in size and length by a rapid multiplying, proliferation, of the cells. In 

 the Pycnogonida, on the contrary, a germinal stripe is never found, but the whole yolk mass is 

 immediately enclosed by a blastoderm, and all the limbs arise, if anything, from the sides of the bla- 

 stoderm bv a segmentation of corresponding parts of the blastoderm with enclosed yolk mass. Further- 

 more onlv the three first pairs of limbs, the embryonal legs, are formed at the same time, while the 

 following four pairs, the ambulatory legs, are segmented off by degrees from before backward, most 

 frequently one pair after the other and with longer or shorter intervals of time. The ganglia seem 

 to develop contemporaneously with the embryonal limbs, and the ganglia of the abdominal side are 

 divided into two principal sections, a foremost one for the embryonal legs, and a hindmost one for 

 the ambulatory legs; but this latter mass of ganglia is not till a later stage separated into four or 

 five pairs of ganglia by degrees as the ambulatory legs develop. I may refer to my figures pi. I, 

 fig. 11 and pi. II, fig- iS, both representing what I call the second larval stage; the first figure repres- 

 ents Pseudopallene circularise in which the whole mass of ganglia is seen still undivided, and only the 

 nerve mass belonging to the segments of the embryonal legs, has been slightly separated; the other 

 figure represents Nymphon Sluiteri, in which the two pairs of ganglionic centra may be distinctly 

 discerned united to a common mass, while the nerve mass of the first pair of ambulatory legs is well 

 separated from the following mass representing the ganglionic mass of the three following pairs of 

 ambulatory legs, in which mass, however, as yet only two pairs of ganglionic centra are to be seen. 

 The larva of Nympkon Sluiteri upon the whole is more developed than that of Pscudopalh'iie. 



Between the embryonal and the larval stage there is no distinct boundary, in 

 so far as this boundary is to be determined by the embryo's leaving the egg; but the 

 embryo leaves the egg sometimes on an earlier, sometimes on a later stage. 



I have already before mentioned that according to the common view the limit of the embryo- 

 nal stage is formed by the embryo breaking the egg shell or egg membrane, and that the whole 



