188 



THE ACEPHALA. 



^172. 



pletely surrounds the body and has only an oral and anal opening ; '-' and 

 with the compound species, it is continuous with the common substance which 

 contains the individuals and binds them into more or less regular groups, 

 and is, therefore, analogous to a corallum. With the Lamellibranchia, and 

 Brachiopoda, it is more or less open, or even may be wholly divided into 

 halves ; ^■'' it has here the property, especially upon its borders, of secreting 

 calcareous matter for the formation of the shell. 



§ 172. 



With the Tunicata, the mantle is remarkable both for its histological 

 structure, and its chemical composition. Recent investigations have shown 

 that, with the Ascidiae and Salpinae, it is composed of Cellulose and there- 

 fore of a non-azotized substance. ''* 



Its anatomical structure is quite complicated. Usually it can easily be 

 separated into two or three layers, the internal one of which is composed, in 

 some species, of a lamellated epithelium formed of a single layer of poly- 

 gonal nucleated cells. <-^ 



Its principal mass in both the compound and simple forms of this order, 

 is formed of a single, or a double confluent layer of a homogeneous trans- 

 parent substance, through which are scattered granules, nuclei, groups of 

 pigment molecules, cells, fibres, and crystals of carbonate of lime, — all 

 varying according to genera and species, and often differently arranged in 

 one and the same species.'-'* But in each species, they are variously ar- 

 ranged in the inner portion of this mantle-substance.'^* In some species 



2 These openings are properly only simple ori- 

 fices of the cavity of the body, and correspond to the 

 respiratory tubes of certain Lamellibrancliia ; see 

 below, § 190. 



3 With Mya, Panopaea, Pholas, Teredo, As- 

 pergillum, the mantle is almost entirely closed, 

 but it has two long fissures at each extrem- 

 ity with So/era, Cyc/as, Tellina, Mytitus, Litho- 

 domus and others; with the Ostracea, Pectinea, 

 Arcacea, Naiades, and Brachiopoda, it is entirely 

 open. 



1 This important fact was first stated by Carl 

 Schmidt (Zur. vergleich. Physiol, d. wu-bellosen 

 Thiere. 1845, p. 61), with Cynthia mamillarix, 

 and has subsequently been confirmed by Lijwig 

 and Kolliker, after the most careful investigations 

 upon the entire order of Tunicata (Compt. rend. 

 1846, p. 38). These two authors found this non- 

 azotized substance, particularly in the different spe- 

 cies of Phallusia, Cynthia, Clavelina, Diazona, 

 Botryllus, Didemnum, ApUdium, Sa/pa, and Py- 

 rosoma ; but not with the other MoUusca, nor 

 with the Annelides, the Hehninthes, the Echino- 

 dermata, the Acalephae, and the Polypi. It is 

 certainly wanting in the true Infusoria, for Frus- 

 tulia salina, which Cari Schmidt cites as be- 

 longing to tliis order and as containing cellulose, 

 is evidently a vegetable. Lowig and KolUker 

 justly fear, moreover, that this discovery will be 

 quickly seized by those who deny that there is any 

 limit between the animal and vegetable kingdom 

 (see loc. cit. p. 8). They seek, therefore, to oppose 

 this view by insisting upon the circumstance that 

 this cellulose is never found in a pure state-in the 

 mantle of the Tunicata, but always combined with 

 other substances, and that, moreover, no animal 

 has as yet been found entirely composed of this 

 substance. 



2 Phallusiamamillans, sulcata, Cynthiapapil- 

 lata, pomaria, and Salpa bicaudata. 



3 This basement substance is homogeneous, and 

 has the same chemical properties as cellulose. 



4 KolUker has made very detailed investiga- 

 tions upon the structure of this mantle. He has 

 kindly allowed me to communicate his results, and 

 authorized me to make use of them without waiting 

 for the publication of his work in common with 

 Lijwig (Ueber das Vorkommen von Ilolzfaser im 

 Thierreich). According to them, the middle layer 

 of the mantle of Phallusia monachus, and sul- 

 cata, Clavelina lepadiformis, and Aplidium 

 gibbulosum, contains numerous nuclei and star- 

 like crystals lodged in a transparent structureless 

 substance. But the external layer of this organ is 

 filled with very large round cells with very thin 

 walls, containing no nueleus, but filled with a 

 transparent liquid. With Clavelina lepadifor- 

 mis, the peduncle and branches of the whole man- 

 tle are so crowded with non-nucleated cells, some 

 round and others elongated, that the basement- 

 substance is apparently absent. It has therefore 

 quite the aspect of a vegetable tissue. With 

 Aplidium gibbulosum, and Botryllus violaceus, 

 the cells of the external layer contain carbonate of 

 lime which ultimately so increases that it gives 

 them a petrified aspect. With Didemnum candi- 

 dum, these petrified cells have calcigerous rays 

 and are so numerous that the whole mass of this 

 compound Ascidian appears filled with white star- 

 like corpuscles. 



According to Milne Edwards, this is true also 

 of Leptoclinum maculosvm (Observ. sur les As- 

 cidies composees, p. 81, i'l. VIII. fig. 2''). 



With Diazona viotaceu7n, Pyrosoma gigan- 

 teum, Botryllus polycyclus, Salpa maxima, and 

 bicaudata, the mantle is without these elegant cells, 

 and in the basement-substance are found only 

 granules and nuclei, and with Diazona, in addi- 

 tion, are i>igment-granules, and crystalline points, or 

 calcareous concretions. 



