T04 DESCRIPTIONS OF PREPARATIONS. 



walls. This atrial cavity is produced by two invaginations from the exterior 

 in the larva, the openings of which unite, forming the single atrial aperture. 

 The atrial must not be confounded with the body cavity which, if present 

 at all, is probably represented by blood spaces in the body walls, &c. 



The stomach is posterior and ventral : the intestine rises from its 

 anterior ventral end, runs forwards for a short space, and then turns back 

 upon the stomach dorsally and posteriorly. It passes to the left side of 

 the aperture of the pharynx into the stomach, and then turns forwards and 

 dorsally to end in the anus, which may be seen just above the dorsal 

 edge of the pharynx internally to the atrial aperture. The cavity of the 

 intestine is partially divided by a ridge or typhlosole which projects from its 

 right wall, and commencing at the pyloric end of the stomach, extends in 

 this Ascidian to within a short distance of the anus. In the substance of the 

 body walls, which are left along the ventral edge of the intestine close to 

 its origin, and in the concavity of its last curve may be noticed some opaque 

 white cords. These are probably portions of the testis. 



The substance of the test is sometimes fibrillated : it contains cellular 

 elements which wander into it from the ectoderm. These cells increase the quantity 

 of matrix. Some of them may become pigmented and others frequently undergo 

 extreme vacuolation, becoming little more than bladders, with a thin wall of 

 protoplasm which contains the nucleus. Chemically, the matrix appears to be 

 identical with the cellulose of plants. 



The terms ' mantle ' and ' branchial sac,' often used in speaking of the body 

 walls and pharynx respectively, are better discarded, as it is a well-established fact 

 that there is no relationship between the Urochorda and Mollusca. 



The muscle fibres are unstriped and have the form of fusiform or filiform 

 fibres. There is a well-developed sphincter both at the oral and atrial apertures, 

 and the test at these spots is generally lobed eight lobes round the oral, six round 

 the atrial, aperture of the Ascidiidae. The part of the test between the thickened 

 lobes is very thin and bendable. The mode in which the muscle fibres are 

 disposed in the body walls is characteristic in the sub-groups of Urochorda. The 

 connective tissue corpuscles of the body walls are often pigmented. 



The tentacles spring from a ridge which coincides with the line in the oral 

 aperture where the test ends. They are simple and filiform. Posteriorly to the 

 circle of tentacles is a plane area the praebranchial zone limited behind by two 

 ciliated ridges bounding a ciliated groove. The ridges are the two peripharyngeal 

 bands. The posterior is continuous ventrally with the ridges bordering the 

 endostyle, and the groove of the latter is continuous with the peripharyngeal 

 groove. The posterior is also continuous dorsally with the dorsal lamina. The 

 anterior band forms a complete circle. 



The cavity of the pharynx is lined with endoderm. Its outer or atrial 

 surface is covered by the invaginated ectoderm. The stigmata lying between 

 the transverse vessels and the fine longitudinal vessels are usually straight, but 

 sometimes curved. Under some conditions it appears that the direction of the 

 ciliary current may be reversed, and water be expelled at the oral instead of the 



