Organisation of an Ascidia (as in Fig. 220, seen from the left). Fig. 221—Organisation of an Ascidia (as in Fig. 220, seen from the left). _sb_ branchial sac, _v_ stomach, _i_ small intestine, _c_ heart, _t_ testicle, _vd_ sperm-duct, _o_ ovary, _ o_′ ripe ova in the branchial cavity. The two small arrows indicate the entrance and exit of the water through the openings of the mantle. (From _Milne-Edwards._) There is no trace in the fully-developed Ascidia of a chorda dorsalis, or internal axial skeleton. It is the more interesting that the young animal that emerges from the ovum _has_ a chorda, and that there is a rudimentary medullary tube above it. The latter is wholly atrophied in the developed Ascidia, and looks like a small nerve-ganglion in front above the gill-crate. It corresponds to the upper “gullet-ganglion” or “primitive brain” in other vermalia. Special sense-organs are either wanting altogether or are only found in a very rudimentary form, as simple optic spots and touch-corpuscles or tentacles that surround the mouth. The muscular system is very slightly and irregularly developed. Immediately under the thin corium, and closely connected with it, we find a thin muscle tube, as in the worms. On the other hand, the Ascidia has a centralised heart, and in this respect it seems to be more advanced than the Amphioxus. On the ventral side of the gut, some distance behind the gill-crate, there is a spindle-shaped heart. It retains permanently the simple tubular form that we find temporarily as the first structure of the heart in the vertebrates. This simple heart of the Ascidia has, however, a remarkable peculiarity. It contracts in alternate directions. In all other animals the beat of the heart is always in the same direction (generally from rear to front); it changes in the Ascidia to the reverse direction. The heart contracts first from the rear to the front, stands still for a minute, and then begins to beat the opposite way, now driving the blood from front to rear; the two large vessels that start from either end of the heart act alternately as arteries and veins. This feature is found in the Tunicates alone. Of the other chief organs we have still to mention the sexual glands, which lie right behind in the body-cavity. All the Ascidiæ are hermaphrodites. Each individual has a male and a female gland, and so is able to fertilise itself. The ripe ova (Fig. 221 _o_′) fall directly from the ovary (_o_) into the mantle-cavity. The male sperm is conducted into this cavity from the testicle (_t_) by a special duct (_vd_). Fertilisation is accomplished here, and in many of the Ascidiæ developed embryos are found. These are then ejected with the breathing-water through the cloaca (_q_), and so “born alive.”