52 



SPONGES 



out involving the surrounding protoplasm (fig. 24 c-f). The result- 

 ing nuclei at length cease to exhibit a nucleolus, and become directly 

 transformed into the heads of spermatozoa; the tails are appropriated 

 by each head from the common protoplasmic residue. The mother- 

 cell in this case undergoes no increase in volume as development 

 proceeds, and it is not enclosed within an " endothelial " layer. In 

 the second and apparently more usual case (20) no "cover- 

 cell " is formed, but the mother-cell divides and subdivides, 

 protoplasm as well as nuclei, till a vast number of minute 

 cells results ; the nucleus of each becomes the head of a 

 spermatozoon and the protoplasm its tail. In this case the 

 sperm-ball does increase in bulk : it grows as it develops, 

 and the cavity containing it becomes lined by epithelium, 

 or so-called "endothelium" (fig. 24/). No doubt (75) the 

 development of the epithelium stands in direct physiological 

 connexion with the growth of the sperm-ball. 



Ccelenterate history as exemplified in the last two events will furnish 

 an explanation of the remarkable divergencies which distinguish 

 the two phyla. The history of the second or planula type has been 

 thoroughly worked out by Schulze (20) in a little incrusting Tetrac- 

 tinellid sponge (Plakina monolopha, Sclmlze). The ovum by regu- 

 lar segmentation produces a blastosphere, the blastomeres of which 



^ *?t * 



9&W* 



-,is J tf^g^4S 

 8Sgpftfiss?asJ9: 



Embryo- Obscure as are the details of this subject, suffi- 

 lo S7- cient is known to enable us to make out two chief 



types of development. One, common amongst the 



calcareous sponges, and possibly occurring in a single 



genus (Gwnmina) of the Micromastictora, is char- 

 acterized by what is known as the " amphiblastula " 



stage; the other, widely spread amongst the 



Micromastictora (Seniera, Desmacidon, Eiispongia, 



Spongelia, Aplysilla, Oscarella), is characterized by 



a " planula " stage. 



The first has been most thoroughly investigated in 

 Grantia raphanus by Schulze (20). The ovum by repeated 

 segmentation gives rise to a hollow vesicle, the wall of 

 which is formed by a single layer of cells blastosphere 

 (fig. 25 d). Eight cells at one pole of the blastosphere 

 now become differentiated from the rest ; they remain ^j 

 rounded in form, comparatively large, and become filled 

 with granules (stored nutriment), while the others, rapidly 

 multiplying by division, become small, clear, columnar, 

 and flagellated. By further change the embryo becomes 

 egg-shaped; the granular cells, now increased in number 

 to thirty-two, form the broader end, and the numerous ^ 

 small flagellated cells the smaller end. Of the granular 

 cells sixteen are arranged in au equatorial girdle adjoin- 

 ing the flagellate cells. A blastosphere thus differen- 

 tiated into two halves composed of different cells is " 

 known as an amphiblastula. The amphiblastula (fig. 25 c] 

 now perforates the maternal tissue, and is borne along an i = 

 excurrent canal to the oscule, where it is discharged to FIG. 26. Development of a Dcmespmv}ia.(riaki,ia monoloplia). n, planula (the central part 



the exterior and swims about in a whirling lively dance. 

 It then assumes a more spherical form, a change premoni- 

 tory of the next most remarkable phase of its career. In 

 this the flagellated layer becomes flattened, depressed, and 

 finally invaginated within the hemisphere of granular cells, 

 to the inner face of which it applies itself, thus entirely obliterating 

 the cleavage cavity, but by the same process originating another 

 (the invagination cavity) at its expense (fig. 25/). The two-layered 

 sac thus produced is a paragastrula ; its outer layer, known as the 

 epiblast, gives rise to the ectoderm, the inner layer or hypoblast to 

 the endoderm. The paragastrula next becomes somewhat beehive- 

 shaped, and the mouth of the paragastric cavity is diminished in 

 size by an ingrowth of the granular cells around its margin. The 

 larva now settles mouth downwards on some fixed object, and ex- 

 changes a free for a fixed and stationary existence (fig. 25 3). The 

 granular cells completely obliterate the original mouth, and grow 

 along their outer edge over the surface of attachment in irregular 

 pseudopodial processes, which secure the young sponge firmly to 

 its seat (fig. 25 h). The granular cells now become almost trans- 

 parent, owing to the exhaustion of the yolk granules, and allow 

 the hypoblast within to be readily seen ; a layer of jelly-like 

 material, the rudimentary mesoderm, is also to be discerned between 

 the two layers. The spicules then become visible ; slender oxeas 

 appear first, and afterwards tri- and quadra-radiate spicules. The 

 larva now elongates into a somewhat cylindrical form ; the distal 

 end flattens ; and an oscule opens in its midst. Pores open in the 

 walls ; the endodermal cells, which had temporarily lost their 

 flagella, reacquire them, at the same time extending the character- 

 istic collar. In this stage (fig. 25 h, j) the young sponge corresponds 

 to a true Ascon, no trace of radial tubes being visible ; but as they 

 characterize the parent sponge they must arise Inter, and thus we 

 have clear evidence through ontogeny of the development of a 

 Sycon sponge from an Ascon. 



The three most striking features in the history of this larva are, 

 first, the amphiblastula stage ; next the invagination of the llagi'l- 

 late cells within the granular, instead of invagination in the reverse 

 order ; and third the attachment of the larva by the oral instead of 

 the aboral surface. Should Srhulze be corrsct in deriving the 

 sponges from the Cuilcntcra, it is probable that the reversal of the 



should be shaded), b, Section through side of planula ; ec, flagellated cells ; ft, their 

 flagella ; col, crenoblast. c, Attached gastrula (the paragaster is formed by fission), d, 

 Section across the foregoing, e, Young sponge (Rhagon). /, Part of a section through 

 fully grown sponge ; the attached basal layer is the hypomere ; the spongomere is fnM.il 

 so as to produce incuirent and excurrent canals ; the canal system is eurypylous ; ov, ova 

 (a segmented ovum lies between two of them) ; bl, blastospheres. After Schulze. 



increase in number by further subdivision till they become con- 

 verted into hyaline cylindrical flagellated cells (fig. 26/). Thus a 

 blastosphere is produced consisting wholly of similar flagellated cells. 

 It becomes egg-shaped, and, hitherto colourless, assumes a rose-red 

 tint, which is deepest over the smaller end. The larva (now a 

 planula, fig. 26 a, by the filling in of the central cavity) escapes from 

 the parent and swims aliout broad end foremost. In this stage 

 thin sections show that the cleavage cavity is obliterated, its place 

 being occupied by a mass of granular gelatinous material contain- 

 ing nuclei (fig. 26 b). In from one to three days after hatching the 

 larva becomes attached. It then spreads out into a convex mass, 

 and a cavity is produced within it by the splitting of the central 

 jelly (fig. 26 c, d ; compare Eucope and others amongst the Cceleu- 

 terates). This cavity becomes lined by short cylindrical cells (endo- 

 derm), while the flagellated cells of the exterior lose their flagella 

 and become converted into piiiiiaeorvtes (ectoderm). The gelatin- 

 ous material left between the two layers now formed acquires the 

 characters of true collenchyme and thus becomes the mesoderm. 

 The endoderm then sends off into the mesoderm, as buds, rounded 

 chambers, which communicate with the paragastric cavity by a 

 wide mouth and with the exterior by small pores (fig. 26 c). An 

 oscule is formed Inter, and the sponge enters upon the lihngon phase. 

 Subsequent foldings of the sponge-wall give rise to a very simple 

 canal system (fig. 26/). In addition to these two well-ascertained 

 modes of development others have been described which at present 

 appear aberrant. In Oscarella lobularis, O.S. (sf), a curious series 

 of early developmental changes results in the formation of an 

 irregular paragastrula, the walls of which become folded (while still 

 within the parent sponge) in a complex fashion, so as to produce a 

 form in which the ineurrent and excurrent canals appear to be 

 already sketched out before the flagellated chambers are diflcrenti- 

 atecl off. In Spougilla Gotte describes the ectoderm as becoming 

 entirely lost on the attachment of the larva, so that the future 

 sponge proceeds from the endoderm alone. As Hfmujilla, however, 



