In this connection the amphiblastula larva presents some points of interest. Does this larva retain the characters of an ancestral type of the Spongida, and if so, what does its form mean? It is, of course, possible that it has no ancestral meaning but has been secondarily acquired; but, assuming that this is not the case, it appears to me that the characters of the larva may be plausibly explained by regarding it as a transitional form between the Protozoa and Metazoa. According to this view the larva is to be considered as a colony of Protozoa, one-half of the individuals of which have become differentiated into nutritive forms, and the other half into locomotor and respiratory forms. The granular amœboid cells represent the nutritive forms, and the ciliated cells represent the locomotor and respiratory forms. That the passage from the Protozoa to the Metazoa may have been effected by such a differentiation is not improbable on a priori grounds.
While the above view seems fairly satisfactory for the free-swimming stage of the larval sponge, there arises in the subsequent development a difficulty which appears at first sight fatal to it. This difficulty is the invagination of the ciliated cells instead of the granular ones. If the granular cells represent the nutritive individuals of the colony, they, and not the ciliated cells, ought most certainly to give rise to the lining of the gastrula cavity, according to the generally accepted views of the morphology of the Spongida. The suggestion which I would venture to put forward in explanation of this paradox involves a completely new view of the nature and functions of the germinal layers of adult Spongida.
It is as follows:—When the free-swimming ancestor of the Spongida became fixed, the ciliated cells by which its movements used to be effected must have to a great extent become functionless. At the same time the amœboid nutritive cells would need to expose as large a surface as possible. In these two considerations there may, perhaps, be found a sufficient explanation of the invagination of the ciliated cells, and the growth of the amœboid cells over them. Though respiration was, no doubt, mainly effected by the ciliated cells, it is improbable that it was completely localized in them, but they were enabled to continue performing this function through the formation of an osculum and pores. The collared cells which line the ciliated chambers, or in some cases the radial tubes, are undoubtedly derived from the invaginated cells, and, if there is any truth in the above suggestion, the collared cells in the adult sponge must be mainly respiratory and not digestive in function, while the epiblastic cells, which in most cases line the inhalent passages through its substance[69], ought to be employed to absorb nutriment. The recent researches of Metschnikoff (No. [134]) on this head shew that the nutriment is largely carried into the mesoblast cells, which in Sycandra appear to be derived from the granular cells, and also that it is taken up by the cells which line the passages, though not by the superficial epiblast cells. Whether the collared cells generally absorb nutriment is not clear from his statements: but he finds that they do not do so in Silicispongiæ.
Professor Schulze has informed me by letter that he finds the collared cells to be respiratory in function, while the cells derived from the granular cells in Sycandra are nutritive. Carter[70], on the contrary, from his observations on Spongilla, has fully satisfied himself that the food is absorbed by the cells lining the ciliated chambers.
If it is eventually proved by further experiments on the nutrition of sponges, that digestion is mainly carried on by the general cells lining the passages and the mesoblast cells, and not for the most part by the ciliated cells, it is clear that the epiblast, mesoblast and hypoblast of sponges will not correspond with the similarly named layers in the Cœlenterata and other Metazoa. The invaginated hypoblast will be the respiratory layer and the epiblast and mesoblast the digestive and sensory layers; the sensory function being probably mainly localized in the epithelium on the surface, and the digestive one in the epithelium lining the passages and in the mesoblast. Such a fundamental difference in the primary function of the germinal layers between the Spongida and the other Metazoa, would necessarily involve the creation of a special division of the Metazoa for the reception of the former group.
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(124) Ganin[71]. “Zur Entwicklung d. Spongilla fluviatilis.” Zoologischer Anzeiger. Vol. I. No. 9, 1878.
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[67] Metschnikoff was the first to give this account of the development of the spicules in Sycandra, but Prof. Schulze has informed me by letter that he has arrived at the same result.
[68] Keller (No. [129]) has recently given an account of the development of Halichondria (Chalinula) fertilis. He finds that there is an irregular segmentation, followed by a partial epibolic invagination, the inner mass of cells remaining exposed at one pole and forming there a prominence, equivalent to the granular prominence in the larvæ of other Silicispongiæ. The free-swimming larva resembles the larva of other Silicispongiæ in the possession of spicula, etc., and after becoming laterally compressed attaches itself by one of the flattened sides. A central cavity is formed in the interior with ciliated chambers opening into it, and is subsequently placed in communication with the exterior by the formation of an aperture which constitutes the osculum.
[69] That the greater part of the flat cells which line the passages of most Sponges are really derived from epiblastic invaginations appears to me to be proved by Schulze’s and Barrois’ observations on the young fixed stages of Halisarca. Schulze’s (No. [140]) observations have however proved that the flat cells lining the axial gastric chamber of Sycandra are hypoblastic in origin, and the observations of Keller (No. [129]) and Ganin (No. [124]) have led to the same result for the flat epithelium lining part of the passages of the Silicispongiæ.