PYCNOGONIDA. 



2 9 



disappeared second pair of embryonal legs, which is then marked as 3 I in his figures, pi. 16 and 17, 

 fig. 3 12. If Semper had known the development of more Pycnogonid-larvse, his explanation would 

 immediately have appeared impossible to himself. 



Morgan, Con trib. Embryol. 1891, has found in Pallene cmpusa, p. 24seq. the same development 

 as I have found in my two species of that genus. I dare not, however, say of the development of 

 the three foremost pairs of ambulatory legs that it is simultaneously, even if the interval between 

 the second and third pair is only small, as has been already observed. Nor can I, as is seen by the 

 foregoing, agree with Morgan as to the fact that this third pair (i.e. the rudimentary embryonal 

 legs) grow out again to form the ovigerous legs. Finally I do not find that Pallene with Pseudo- 

 pallenc deviates so much from the ordinary Pycnogonid-development, that there is sufficient reason to 

 finish, as does Morgan, the description of its ontogeny in the following way: The development of 

 Pallene has become so much abbreviated that there is only a trace of the true Pantopod-larva found 

 in its ontogeny, cp. the foregoing. 



Dohrn, Bau u. Entwickl. Arthrop. , 1870, p. 144 51, gives a detailed description of Achelia 

 (^{ininotlica} Itcvis, and in pi. VI, fig. u 13 he draws three consecutive stages of the development 

 of this animal, of which three stages I refer his first (in the explanation of the plates called mittleres 

 Stadium) and second stages to my second stage. In the corresponding figures, fig. u and 12, the 

 embryonal legs are still seen fully developed, with the exception that the last pair in fig. 12 are some- 

 what smaller than the other embryonal legs; but judging by my examinations, I regard the smaller 

 size, cp. also my fig. 24 on pi. I , as a consequence of the fact that only the empty sheaths are left, 

 and that besides the points or outermost joints of these sheaths, as in my figure, are retracted, partly 

 into the preceding joint ; I cannot suppose a reduction or real diminution of- this pair of legs to have 

 taken place. -- Also the larvae given by Dohrn, Pantop. Golf. Neap. 1881, pi. XI, fig. 21 and 24, and 

 determined as Phoxichilus vulgaris. must be referred to our second larval stage; but the specific 

 determination, especially with regard to the last figure, seems to me to be very doubtful. The first 

 figure, fig. 21, represents a normal Pycnogonid-development, in which I only think it to be not very 

 probable that a Phoxichilus should have kept its chelifori so long, and not rather have lost them, 

 either by reduction or throwing off, while the nearly related Pycnogonum has wholly lost them before 

 the close of the stage , cp. my figure, pi. I , fig. 4. The improbability that Phoxichilus should have 

 kept the chelifori so long, is of course greater with regard to the older larva, fig. 24, Pycnogonum 

 having lost them on a little earlier stage. But I am still more unable to believe that the imaginal 

 fore limbs, of which only the hindmost pair are developed and kept only in the male, should commence 

 and begin a development which was soon to be stopped or reduced; for I think it a fact that admits 

 of no doubt, that the appendages, marked in fig. 24 with II and III, are not rudimentary, reduced 

 remnants of the embryonal legs, but on the contrary a beginning development of the imaginal 

 fore limbs. 



The third larval stage begins, when the fourth pair of ambulatory legs, which 

 have until now been far behind in development, together with the interjacent caudal 

 segment, begin to grow and develop, until they obtain their permanent shape, the 

 legs resembling the three foregoing pairs. The imaginal fore limbs, palps, and ovi- 



