336 SUMMARY OF CUEBENT EESE ARCHES RELATING TO 



this latter are invested in a delicate chitinous layer. The termination 

 of the oesophagus is not abrupt ; its three inner faces are prolonged 

 towards the interior of the intestine, and give rise to three outgrowths 

 which have all the appearance of special glands ; tubular prolonga- 

 tions are, as is well known, connected with the intestine, but, though 

 they no doubt are very important physiologically, the author has 

 grave doubts as to their morphological significance. 



Great difficulties seem to attend a satisfactory study of the circula- 

 tory system ; the heart has three cavities, at the end of each of 

 which there is a pair of orifices ; it is probable that there is an aorta, 

 although it has not yet been detected ; as the author has mentioned 

 in his ' Challenger ' report, the dorsal surface of the heart is 

 remarkable for having no muscular fibres. 



The sexes may be easily distinguished, for, with rare exceptions, 

 the males carry the fecundated ova. Contrary to what generally 

 happens, the females have lost the primitive organization of the 

 generative organs, while the males have been more conservative. For 

 elaborated details on this, as on various other points, the author refers 

 to his ' Challenger ' report.* 



Dr. Hoek would place the larvae of Pycnogonids with the primary 

 larvae of Prof. Balfour. When we consider the zoological position 

 and classification of the Pycnogonida, we are led to the conclusion 

 that the doctrine of Semper, which regards them as Arachnida, 

 has nothing to defend it ; the only real point of resemblance 

 between them lies in their having the same number of thoracic 

 appendages; the similarity in the formation of the first pair of 

 appendages, lately dwelt upon by Balfour, seems to the author to be 

 of less significance than the fact that this organ is innervated by a 

 nerve arising from the sub-oesophageal ganglion. Dr. Hoek thinks 

 that the Pycnogonida must form a distinct class of the Arthropoda, 

 comparable to the Crustacea, Insecta, &c. 



Starting from the protonymph, or larval form common to Asco- 

 rliynclms, NympJion, and Pi/cnogonum, and noting that in the two 

 former there remain appendages, which become cephalic, while in the 

 last they are during development obliterated, we have to consider 

 Pycnogonum as the least ancient form. The doctrine suggested by the 

 history of the metamorphosis is supported by a study of the nervous 

 system ; in the primitive condition the ventral part of the nervous 

 system is represented by six ganglia, excluding the more or less rudi- 

 mentary abdominal ganglia ; of the six segments corresponding to 

 these ganglia, four are thoracic ; and two, in a more primitive con- 

 dition, belong to the cephalic part. As the mandibles are innervated 

 by the suboesophageal ganglion, we have three pairs of cephalic 

 appendages, and this is what is permanently seen in Ascorhynchus and 

 Nymphon. This possession of three cephalic appendages is, by 

 various evidence, indicated as the primitive arrangement. Nymphon 

 retains this most unchanged, but the number of the joints in its 

 cephalic appendages and the structure of the genital organs forbid us 

 to regard it as the most ancient form now living. A hypothetical 



* Sec this Journal, i. (1881) p. SS(J. 



