NATURE 



49 



THURSDAY, MAY 21, 1891. 



PYCNOGONIDS. 



Den Norske Nordhavs-Expedition, 1876-78. XX. Zoo- 

 logi—Pycnogonidea. Ved G. O. Sars. Med 1 5 Plancher 

 og I Kart. (Christiania : Grondahl & Sons, Bogtryk- 

 keri, 1891.) 



Studies from the Biological Laboratory, Johns Hopkins 

 University, Baltimore : A Contribution to the Em- 

 bryology and Phytogeny of the Pycnogonids. By T. H. 

 Morgan, With Eight Plates. (Baltimore : The Johns 

 Hopkins Press, 1891.) 



THE group of sea spiders, or the Pycnogonidea, was 

 for a long time among the least known, though by 

 no means the least interesting, of the divisions of the 

 marine invertebrates. Linnaeus described a species as 

 a Phalangium, placing it among terrestrial forms, and 

 though a century and a quarter has passed since then, 

 the problem of where to place these Pycnogonids cannot 

 be said to be finally settled. 



Within the last ten years or so, an immense advance 

 has been made in our knowledge of the morphology, 

 anatomy, and embryology of the group, thanks to the 

 labours of Anton Dohrn, who, in 1881, described the 

 forms found in the Gulf of Naples, and of Hoek, who 

 about the same date described the species found during 

 the cruises of the Willem Barents and the Challenger. 

 During all this period opinions varied as to whether 

 these forms should be placed among the Arachnids or 

 the Crustacea, but apparently both the authors just 

 referred to have agreed that the Pycnogonids should be 

 placed with neither, but that they, with the Arachnids 

 and the Crustacea, have come down the stream of 

 evolution in parallel lines. 



To the existing recent memoirs of these Arthropods, 

 the splendid volume just published on the Pycnogonidea 

 found during the Norwegian North Atlantic Expedition, 

 1876-78, by Prof. G. O. Sars, adds, perhaps, from a 

 morphological point of view, the most important of the 

 recent publications on the group ; for, valuable as beyond 

 question are the structural and developmental details, a 

 special knowledge of general morphological detail is also 

 needed for the convenient understanding and classifying 

 of any group. 



The material at Prof. Sars's disposal was very large, and 

 in addition he has made use of collections made by himself 

 during many years back on the coasts of Norway, and 

 also of some few forms sent to him by Dr. A. Stuxberg, 

 which had been found in the Kara Sea during Norden- 

 stciold's expedition. A very great contrast is to be seen 

 on comparing these northern forms with such a collection 

 as that of Dohrn from the Mediterranean. The great 

 number of species belonging to the family Nymphonidae 

 is specially characteristic of the Northern Seas as con- 

 trasted with the Mediterranean, while again the Northern 

 Sea species attain very generally much larger dimensions, 

 some being gigantic in comparison with those of the 

 Mediterranean. 



In working out the classification of the group, Sars has 

 found it necessary to treat the families in a somewhat more 

 NO. I 125, VOL. 44] 



restricted sense than has been done by most of the previous 

 writers, and has been obliged to increase their number. 

 While fully agreeing that the descriptions and even figures 

 of the Pycnogonids given by the earlier writers leave 

 much to be desired, and are as a rule even exceedingly 

 defective, in some cases indeed being so bad as not to be 

 intelligible, yet he thinks that some quite recent describers 

 have rejected as bad a greater number of descriptions 

 than with a little patient research was really necessary. 

 Thus he finds it hard to believe that, while not a few 

 species have been described from the Gulf of Naples, all 

 the species described as found there by Dohrn, with one 

 exception, should be new. Most certainly as regards the 

 northern species we cannot sufficiently admire the pains 

 which Sars has taken in working out all the imperfect 

 descriptions and rough figures of our past recorders of 

 new forms, with the result that he has succeeded in 

 re-establishing many wholly forgotten or ignored species 

 of Goodsir and others. 



As regards the terminology used in describing the 

 various parts, some, classing the Pycnogonids with the 

 Crustacea, adopted terms in use among the latter ; while 

 others, holding their affinity to be with the Arachnids, 

 employed again a different set of terms. Dohrn, to avoid 

 the difficulty as regards the limbs, rejects all special 

 terms, describing them as No. L, IL, &c. ; Sars uses a 

 terminology the terms of which involve as little as 

 possible of any homologous references. 



Forty-three species are described and figured. Several 

 of them are here fully described for the first time, though 

 short diagnoses of them appeared in a preliminary report. 

 The fourteen genera are arranged in eight families, 

 and these are grouped into three orders, the ordinal 

 characters being based on the relations of the " chelifors." 

 Thus in Order i, Achelata, these chelae are, except in the 

 larval state, entirely absent ; in Order 2, Euchelata, the 

 chelae are well developed throughout all the stages of 

 life ; while in Order 3, Cryptochela, the chelae are pre- 

 sent, as a rule, in the young stages (not alone in the 

 larvas), but in the fully developed condition they become 

 atrophied or disappear. This arrangement no doubt will 

 have to be modified so as to fit it to receive the very 

 numerous forms from other parts of the world, but it is a 

 first step in the right direction of an intelligent grouping 

 of the genera. 



The second memoir on our list treats of the Pycnogonids 

 from a different standpoint, being a contribution to our 

 knowledge of the embryology and phylogeny of the group, 

 by T. H. Morgan, Fellow of Johns Hopkins University. 

 After a short allusion to the work of Dohrn and Hoek, 

 who have " placed the morphology of the order on a very 

 firm basis," he proceeds to treat of the early stages of the 

 embryology of the Pycnogonids, stages which have been 

 practically unexamined, and a knowledge of which is 

 needed to enable the relationship of the group to be 

 guessed at. 



The material for this work was collected at Wood's 

 Holl. Three genera, each with a single species, are to be 

 found at this ^IziCQ—Pallene empusa, Phoxichilidium 

 maxillare, and Tanystylum orbiculare ; and during July, 

 August, and September, these were found carrying ova. 

 The alcoholic picro-sulphuric acid process was adopted 

 for hardening; the eggs being cut in paraffin. The 



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