302 



NA TURE 



[May 13, 1909 



eyes develop characteristically in the most intimate 

 connection with optic diverticula from the neural tube? 

 Dr. Gaskell meets this objection by insisting that 

 the retina and optic nerve were originally outside a 

 non-nervous tube — an anterior diverticulum on each 

 side from the alimentary canal — and he remarks : — 



" It is again a striking coincidence to find that 

 Artemia, which with Branchipus represents a group 

 of living crustaceans most nearly related to the 

 trilobites, does possess two anterior diverticula of the 

 •gut which are in extraordinarily close relationship with 

 the optic ganglia of the retina of the lateral eyes on 

 ■each side." 



We are accustomed to think of arthropods as 

 typically provided with a chitinous exoskeleton, and 

 thus contrasted with vertebrates, which have an in- 

 ternal skeleton of cartilage or bone. But Dr. Gaskell 

 shows that this difficulty " vanishes into thin air " 

 before the discovery of the branchial cartilaginous 

 bars of Limulus, together with that of the internal 

 prosomatic plastron. He quotes Schmiedeberg, who 

 pointed out that glycosamine is a bridge between 

 chitin and chondrin. The Palaeostraca were the 

 dominant arthropod race when vertebrates first ap- 

 peared, and " not only had they manufactured an in- 

 ternal cartilaginous skeleton, but they had got it both 

 In structure and position, exactlv at the stage at 

 -which the vertebrate skeleton starts." This almost 

 sounds like proving too much, yet it does not account 

 for the vertebrate's dorsal axis. 



Morphologists are accustomed to lay some emphasis 

 on the branchial clefts of vertebrates, but Dr. Gaskell 

 thinks of the branchial unit as a gill-bearing appen- 

 "dage, and does not hesitate to describe in ammocoetes 

 a respiratory chamber into which a symmetrical series 

 •of sunk-in branchial appendages, the so-called dia- 

 phragms, are dependent. Two large longitudinal 

 venous sinuses in I.imulus correspond to the two veins 

 which come together to form the heart and ventral 

 aorta of the vertebrate. Morphological dogmatism is 

 startled by the homology between the breathing organs 

 in king-crab and lamprey, but it is shocked by the 

 derivation of the thyroid gland from the pateostracan 

 uterus — a derivation the violence of which, as it seems 

 to us, is not lessened by the light it sheds on the 

 mysterious physiological nexus between the sexual 

 organs and the thyroid in man and other animals. 

 The nasal tube of ammocoetes corresponds to the 

 olfactory tube of a scorpion-like animal, and the 

 pituitary body shows by similarity of structure, as 

 well as of position, that it arose from the coxal glands, 

 which were situated at the base of the four endo- 

 gnaths. Special sense-organs, such as are found in 

 the flabellum of Limulus and in the pectens of 

 scorpions, may be looked upon as giving origin to 

 the vertebrate auditory apparatus. Even more sur- 

 prising than these conclusions is the ingenuity of 

 the evidence that the author uses in support of them. 



We cannot follow Dr. Gaskell in his detailed coni- 

 parison of segments, nerves, and musculature in 

 vertebrates and arthropods, but we must direct atten- 

 tion to the twelfth chapter, where the difficulties 

 suggested by the characteristic segmental excretory 

 NO. 2063, VOL. 80] 



organs of vertebrates and by the state of the ccelom 

 in arthropods are dealt with. The author shifts off 

 from the Palaeostraca to the hypothetical Protostraca 

 — ancestral to both arachnids and crustaceans — which 

 possessed in every segment a pair of appendages and 

 a pair of coelomic cavities, each with excretory organs 

 or coxal glands. The hypothetical Protostraca arose 

 from the polychajtes. As to the notochord and the 

 vertebrate gut, the author starts from a trilobite- 

 like animal with a deep ventral groove and pleural 

 fringes ; the groove becomes a tube, and sinks in 

 as the notochord ; a continuation of the same process 

 of ventral groove-formation, combined with the oblitera- 

 tion of appendages and the growth of pleural folds, 

 leads to the closed vertebrate gut. All seems con- 

 sistent with an earthquake-hvpothesis. 



In his extremely interesting fourteenth chapter, Dr. 

 Gaskell shows that the development of a vertebrate, 

 e.g. as regards nerve-tube, branchial skeleton, cranial 

 segments, and excretory organs, reads like a recapitu- 

 lation of the steps which led long ago from arthropod 

 to vertebrate. He also expounds the suggestive view 

 that a very much more important embryological idea 

 than that of the three germinal layers is that which 

 centres the metazoan body in the nervous system, and 

 not in the gut. In the body there are master-tissues 

 — all the neuro-muscular and neuro-epithelial struc- 

 tures — and within the meshes of these there are 

 germ-cells, blood-corpuscles, lymph-corpuscles, con- 

 nective-tissue cells, &c., living a symbiotic existence 

 independent of the central nervous S3'stem. 



The author regrets that his previous publications 

 bearing on the palaeostracan origin of vertebrates have 

 not been adequately criticised. We suppose that this 

 is because the author pays no heed to the conven- 

 tional canons of morphological work. We may say 

 that the known Palaeostraca are much too highly 

 specialised animals to be regarded as plausible 

 starting-points for a new phylum, but the author 

 does not share this view. We may say that the 

 ammocoete is a very peculiar larval chordate type, 

 likely to mislead, and that it is quite illegitimate to 

 ignore the hints offered by Amphioxus and the tuni- 

 cates ; but the author does not agree. The author 

 makes out a seemingly strong case by showing extra- 

 ordinary and unsuspected resemblances between am- 

 mocoete and king-crab, and there is no use criticising 

 these in a general way. The supposed homology of 

 the branchial cartilaginous bars in king-crab and in 

 ammocoete — to take one instance — must be examined 

 in detail by an unprejudiced expert. We wish simply 

 to point out that the ingenious author flits a little 

 from type to type; arachnids are called in where 

 crustaceans will not help ; Peripatus is summoned 

 when the Palaeostraca prove broken reeds ; and, after 

 all, the author takes refuge in the hvpothetical Proto- 

 straca, which have a good deal of the annelid about 

 them. We do not think that the author gets over 

 the difficulties presented by the vertebrate's gill-slits, 

 notochord, coelom, ventral heart, and so on, but we 

 agree that there are difficulties in face of every attempt 

 to affiliate vertebrates to an invertebrate stock. The 

 question is as to which theory presents least difficulty 



