TEGUMENTARY ORGANS. 



493 



layer is formed by the successive deposit of 

 calcareous lamina; inwards. When the first 

 lamina has been formed, a deposition of small 

 cylindrical bodies takes place upon its inner 

 surface. These increase, widen and become 

 ramified at their extremities, forming ramified 

 columns. A second calcareous lamina is now 

 formed, connecting their ramified extremities, 

 upon whose under surface the like process 

 takes place, and this is repeated until the ven- 

 tral layer has attained its full thickness. The 

 ramified columns are regularly transversely 

 striated; with the age of the shell additions 

 are continually made to their lateral dimen- 

 sions until they coalesce and constitute the 

 septa of the perfect shell, upon which the strias 

 remain visible. 



Kolliker was unable to find any cellular 

 structure in the columns or laminae themselves, 

 but describes a layer of nucleated cells under 

 the shell, which he regards as the agents in 

 its secretion. Some researches recently made 

 by H. Miiller (Gegenbaur, Kolliker and H. 

 Miillcr, /. c.) corroborate this view. He finds 

 the shell of the Loligidae invested by an ex- 

 cessively vascular membrane, which is almost 

 wholly covered by a layer of epithelial cells 

 towards the shell. On the dorsal surface they 

 are for the most part rounded ; on the abdo- 

 minal surface, and particularly towards the 

 anterior point, they form narrow cylinders 

 which attain a length of as much as 0'07'". 

 They appear to give rise to the structureless 

 layers of the shell. The lateral styles of the 

 Octopods present similar relations. 



The structure thus described, though appa- 

 rently so widely different from that of ordinary 

 mollusks, does not really differ very widely 

 from the cancellated shell structure of Rudis- 

 tes, &c., or still better of Pleurorhynchus as 

 described by Dr. Carpenter. If we leave 

 out the sides of the hollow prisms in the 

 latter shell in fact, it will correspond exactly 

 with one lamella of the ventral layer of the 

 sepiostaire. 



For a comparison of the shells of Spirula 

 and Belemnites with those of Sepia and of 

 Gasteropods, I must refer to Dr. Carpenter's 

 Memoir so often cited. 



2. Convcrsionary integument of the Mollusca 

 containing cellulose. This form of integument 

 has hitherto been found in the Ascidians alone, 

 in which the existence of cellulose was first 

 detected by Schmidt in 1845. Schmidt's dis- 

 covery was confirmed, the fact of the existence 

 of cellulose in all the genera of Ascidians deter- 

 mined, and the chief morphological characters 

 of their test set forth in the memoirs by Lb'wig 

 and Kolliker " Sur les Enveloppes des Tuni- 

 ciers," which appeared in 1846, since when 

 further investigations have been made by 

 Schacht and by myself. I must refer the 

 reader to these papers for an account of the 

 various opinions which have been entertained 

 with regard to the structure of the Ascidian 

 test, as I can only lay before him what are 

 in my belief the facts of the case. 



The test of the Ascidians is never composed 

 of pure cellulose, hut consists of an animal 



membranous matrix, to which the cellulose 

 has the same relation as the calcareous salts 

 have to the membranous basis of bone or of 

 shell. The cellulose is, in fact, diffused through 

 the membranous matrix, thoroughly impreg- 

 nating it. 



This membranous nitrogenous matrix in 

 which the cellulose is deposited, presents great 

 diversities of structure in the genera of Asci- 

 dians, representing, in fact, almost every known 

 tissue. Thus in one genus we have a test re- 

 sembling cartilage ; in another, like bone ; in a 

 third, like connective tissue. It may either be 

 without vessels or traversed by branched and 

 ramified vascular processes of the body. It 

 is in all cases, however, a product of the 

 metamorphosis of the ecderon of the outer 

 tunic or mantle and, complicated as its struc- 

 ture may be, corresponds morphologically with 

 the shell of other mollusks, or with the epi- 

 dermis of the higher animals. In fact, if a 

 section be made through the outer tunic and 

 test of an Ascidian, as \njig. 314. A, taking care 

 not to disturb the natural relations of the parts, 

 we observe at the line of contact between 

 the outer tunic and the test () an arrange- 

 ment of the parts very closely resembling 

 what exists at the junction of the derma with 

 the rete Malpighii in the human skin. The 

 outer tunic, like the former, is constituted 

 by bundles of rudimentary connective tissue 

 which run inwards to form sheaths around 

 the muscles, leaving between them spaces, 

 the sinuses of the blood vascular system, 

 while externally they fuse together into a 

 homogeneous substance containing endo- 

 plasts, which is thrown into processes and 

 passes insensibly outwards into a layer of 

 similar substance, with very close-set endo- 

 plasts almost perpendicular to its surface, 

 which forms the commencement of the proper 

 test. Externally to this rete Malpighii, the 

 deposit of cellulose commences; the tissue un- 

 dergoing at the same time a fibrous metamor- 

 phosis. The line a is therefore a protomor- 

 phic line, and the test is the product of the 

 growth and conversion, by deposit of cellulose 

 within its elements, of a true ecderon. 



The separation of the ecderon (or test) 

 from the enderon (outer tunic) takes place 

 with great readiness in some Ascidians, as 

 Phallusia,&c. ; while in others, such as Boltenia, 

 many Cynthise, Salpa;, &c., it can be effected 

 with considerable difficulty, or not at all, and 

 this difference has even been raised to the rank 

 of a zoological distinction, the Ascidians having 

 been in consequence divided into Monochito- 

 nida and Dichitonida. I believe, however, 

 that in all those Ascidians whose test is un- 

 provided with vessels, it is, normally, closely 

 adherent to the outer tunic, and I am inclined 

 to think that this is equally true even of those 

 forms, such as the Phallusiae, in which in 

 preserved specimens the test and outer tunic 

 are so commonly found detached from one 

 another. Here, however, as the test is pro- 

 vided with an abundant vascular supply pro- 

 ceeding from one point of the body, it may 

 normally become separated elsewhere. Care- 



