562 



NA TURE 



[OCTOUER 8, 1896 



considered as homologous lo ihe crustacean green glands. In a 

 precisely corresponding position, and presenting a structure 

 fairly similar to that of the coxal gland of Limulus, we find in 

 Ammocoetes and in other vertebrates the pituitary gland. How 

 far this gland tissue is developed in connection with the nian- 

 diliular head cavity I do not know, but I venture to suggest that 

 the conijilete evidence of its homology w ith the coxal gland will 

 be found in its developmental connection with the walls of the 

 2nd or mandibular head cavity. 



Comfurisoti of Ihf J'crhiirah' Hear/ ami Vriifrai Aoiia with the 

 I'liitral Longiltiilinal Bramhial Siiiiisis .'/ /.iniiiliis and its 

 Attics. 



The heart of the vertebrate presents two striking peculiarities, 

 which make it different from all invertebrate hearts; first, its 

 developmental history is different ; and, secondly, it is at first 

 essentially a branchial rather than a systemic heart. The re- 

 searches of Paul Mayer (Milt/i. a. d. Zool. St. :u Neapel, 

 vol. vii.) have shown that the subintestinal vein, from which in 

 the fishes the heart and ventral aorta arise, is in its origin double, 

 so that in all vertebrates the heart and ventral aorta arise from 

 two long veins which arc originally situated on each side of the 

 middle line. By the formation of the head fold these come 

 together ventrally, coalesce into a single tube to form the sub- 

 intestinal vein and heart, still remaining double as the tw'o 

 ventral aortK with their branchial branches into each gill, as is 

 well shown in the case of Ammocretes. 



It is a striking coincidence that in Limulus and the Scorpions 

 two large venous collecting sinuses are found situated in the 

 same ventral position, for the same purpose of sending blood to 

 the branchiie, as already described for the vertebrate ; still more 

 striking is it to find, according to the researches of Milne 

 Edwards and Blanchard, that these longitudinal sinuses have 

 already begun to function as branchial hearts, for they are con- 

 nected with the pericardium by a system of transparent muscles, 

 described by Milne Edwards and named by Lankester veno- 

 pericardiac muscles. These muscles are hollow, both near the 

 vein and near the pericardium, so that the blood in each case 

 fills the cavity, and, as they contract with the heart, that part 

 of them in connection with the venous collecting ^inus already 

 functions, as pointed out by Milne Edwards and Blanchard, as a 

 bianchial heart. 



By this theory, then, even the formation of the vertebrate 

 heart is prevised in Limulus, and I venture to think that in 

 Ammocoetes we see the remnant of the old dorsal single heart 

 of the arthropod in the form of that peculiar elongated organ 

 composed of fattily degenerated tissue which lies between the 

 spinal cord and the dorsal median skin. 



Comparison of the Cnticiilar and Laminated Layers of the Skin 

 of Avinioceetes with Chitinoiis Layers. 



The external epithelial cells of Animoccetes possess a remark- 

 ably thick cuticular layer. The striated appearance of this 

 layer is due to a number of pores through which the glandular 

 contents of the cells are poured when the surface is made to 

 secrete. That this striated appearance is due to true porous 

 canals, just as in chitin, and not to a series of rods, is easily 

 seen by the inspection of sections, and also by w-atching the 

 secretion through them of rose-coloured granules when the 

 living cell is stained with methylene blue. The surface layer of 

 this cuticular layer, according to Wolff (fen. Zeitsehr., vol. 

 xxiii. ) resists reagents in the same manner as chitin. 



Internal to the epithelial cells of the skin of Ammocreies is a 

 remarkable layer of tissue, generally called connective tissue. 

 It resembles, however, histologically, in the Animoccetes, a sec- 

 tion through chitin most closely ; the layers are perfectly regular 

 and parallel ; cells are found in it with great sparseness, and it 

 is not until after transformation, when it is altered and invaded 

 by new cell elements, that it can be looked upon as at all re- 

 sembling connective tissue. It resembles chitin in its reaction 

 to hypochlorite of soda. In order to completely dissect off this 

 laminated layer from an Ammoccetes, all that is necessary is to 

 place the animal in a weak solution of hypochlorite of soda, and 

 in a short time it entirely disappears, bringing to view the 

 muscles, branchial cartilages, pigment, front dorsal part of the 

 central nervous system, &c., in a most striking manner. At 

 present I am puzzled that so manifest a chitinous covering should 

 lie internal to the epithelial cells of the surface ; such a po.sition 

 is not, however, unknown among invertebrates, and may be 

 •accounted for in various ways. 



NO. 1406, VOL. 54] 



For the sake of clearness I will sum up before you in the form 

 of a table the corresponding parts in Amniocietes and in Limulus 

 and its allies, as far as I have discussed them up to the present, 

 from which ycju will see that there is not a single organ which is 

 ]5resent in the jirosomatic and mesosomatic regions of Limulus and 

 its allies which is not found in the corresponding situation and 

 of corresponding structure in Ammocretes. 



Ta/'/e of Coincidences between IJmultis and its Allies, and 



between Ammocn'les and Vertebrates. 

 LiMtiLUs AMI ITS Allies. Ammociktes and Vertebrates. 

 Centra! Nervous System. 



Supra-cesophageal ganglia Cerebral hemispheres. 



Optic part Optic th.alami, ganglia habenulx. 



&c. 



Olfactory part Olfactory lobes. 



(Esophageal commissures Crura cerebri. 

 Infra-resophageal ganglia Epichordal brain. 



I'rosomatic ganglia ... Ilind brain, cerebellum, post- 



corp. quadrig. 

 Mesosomatic ganglia ... Medulla oblongata. 

 Ventral chain. 



Metasomatic ganglia ... Spinal cord. 

 Alimentary Canal. 



Cephalic stomach Ventricular cavities of brain. 



Straight intestine Central canal of spinal cord. 



Terminal part Neurenteric canal. 



Qisophagus Infundibular tube and saccus 



vasculosus 



Mouth lube Hypophysial lube, later nasal 



canal. 



Liver Part of subarachnoideal glandular 



tissue. 

 Appendages and Appendage 

 Neii>es. 

 Prosomatic or locomotor 



appendages Appendages of oral chamber or 



stomatodaunt. 

 Foremost appendages... Upper lip and tentacles. ' 



Last appendages Velar a]3pendage and median 



ventral tentacle. 



Metastoma Lower lip. 



Nerves of prosomatic ap- 

 pendages Various branches of Vth nerve. 



Mesosomatic or branchial 



appendages Appendages of branchial chamber. 



Opercular appendages Appendage innervated by Vllth 

 nerve. 

 Genital part Thyroid gland and pseudo-bran- 

 chial groove. 



Branch, part Hyobranchial. 



Basal part Septum of stomatodceum. 



Branchial appendages... Branchial appendages innervateJ 

 by I.\'th and .\th nerves. 

 Special Sense Organs and 

 Nerves. 

 Lateral eyes and optic 



nerves Lateral eyes and optic nerves. 



Median eyes and nerves... Pineal eyes and nerves. 

 Camerostoma and olfactory 



nerves Olfactory organ and Ist nerve. 



Flabellum and nerve ... Auditory organ and Vlllth nerve. 

 Epimeral nerves to surface 

 of prosoma and meso- 



soma Sensory part of Vth nerve. 



Internal and E.xternal 

 Skeleton. 

 Internal skeleton. 



Branchial cartilages ... Branchial cartilages. 

 Entapophysial cartilag- 

 inous ligaments ... Subchordal cartilaginous liga- 

 ments. 

 Fibro-mas.sive tissue 

 (forerunner of carti- 

 lage or " Vork- 



norpel") Mucocartilagc or " Vorknorpel." 



External .skeleton. 



Chitinous layer Cuticular layer on surface of body 



and subepithelial laminated 

 layer. 



