56 LOWER INVERTEBRATES. 



wliicb, jn-otnuling below, peneti-ate deeply into the mud, and may either serve as 

 anchors or bases of support. The most curious case of tliis kind occurs in Tethya 

 gravata, a globular form, in which the threads form a network below, enclosing small 

 stones and gravel. Thus tlie animal carries ballast, aud if turned bottom up in the 

 water it rights itself immediately. Wlien rolled over by the waves upon the muddy 

 bottoms of Buzzard's Bay, where it occurs, it is always sm-e to eud its gyrations 

 right side up like a bit of leaded pith. 



The observations of Sehultze on the young of Si/candni, one of the calcisjjonges, 

 show that the ciliated cells, when invaginated, form an ampuUaceous sac, confirming 

 the A'iew that we have always held tliat the typical sponge was a single, isolated 

 ampulla, surrounded by the two layers of the body. A single jjore is opened into this 

 sac, and this completes the likeness to one of the Ascones group. The observations 

 of Barrois, Carter, Sehultze, and Marshall all seem to show that the ampullas in the 

 silicious sponges have a different development. After the larva has settled, a hollow 

 sj)ace appears in the body of the sponge, lined by a non-ciliated endoderm. The 

 ampuUaceous sacs ai-ise as buds from this endoderm, communication with the exterior 

 is formed by tubes, which arise as invaginations of the ectoderm, and grow inward, 

 uniting with the ampulla?. 



The evidence at present seems to be in favor of Bari-ois' opinion, that the water 

 flows in througli these lateral pores and accumulates in the interior, assisting to raise 

 the soft tissue into a dome or spire, until, at last, unable to •withstand the pressure, 

 the top gives way, and the crater is formed. This accounts for the rise of the spire 

 before the formation of the crater, and gives a reason for its disappearance after the 

 pressure has been relieved by the fonnation of an adequate outlet. Certain it is that 

 the crater is not in any sense the mouth or blastopore of the sponge, as is usu.ally sup- 

 posed. Thus the cloacal apertures have no special morphological location, and arise 

 as purely mechanical necessities, as do the excurrent openings of all colonial forms. 



The simplest sponges have only a single body cavity, surrounded by ectoderm and 

 mesoderm, and lined by the inner layers. This tv-pical form or vase shape occurs in 

 the young of the calcareous sponges and in the adidts of the Ascones. Individuation in 

 these forms is complete and simple ; they are each equivalent to a single ampuUaceous 

 sac, se])aratcd from any other sponge and surrounded by mesoderm and ectoderm. It 

 is evident, therefore, that when a number of these sacs still remain connected with the 

 Ijody cavity, each additional sac must be regarded as a bud or offshoot from the 

 cceloniatic cavity, and the whole can be regarded as a branching gastro-vascular system, 

 through which water and food are circulated and excrement discharged. 



The active collared cells of the ampidlie are both structurally and functionally, as 

 was jiointed out by H. James Clark, similar to the zoons of the flagellated Protozoa ; 

 they have the same organization, catch their food by means of the same slender lasl), 

 swallow it at the same place within the collar, aud throw out the refuse matter 

 in precisely the same manner. The Flagellata are individuals, each having the 

 tyjucal structure of the Protozoa, and though in every respect simple cells, wiXh 

 collai-s and flagella, as in the separate cells of the sponge, they are not shut up in sacs 

 inside of a mass of flesli, but are free or attached animals, getting their food in the 

 o])en water. This correlation and the aspect and functions of the cells which form 

 the tissues of all structures in the bodies of sponges and higher animals show us that 

 all cellular tissues must be regarded as aggregates or colonies, while the single cells of 

 which the tissues are comjiosed are the exact morphological representatives of the 



