EXTERNAL CHARACTERS.—Amphioxus[112] is about 1½ to 2½ inches in length, slender, somewhat translucent, and pointed at both ends (Fig. 69). It lives in shallow water and burrows in the sand, head first, with great rapidity. It frequently remains with the anterior end protruding from the sand. When on the surface it lies on one side. It is said to swim freely at night. The head end is rather the thicker, and the anterior two-thirds of the ventral surface are flattened (Fig. 70, A), and may be slightly ridged longitudinally. The lateral edges of this flat area project as metapleural folds (Fig. 70, _mt.pl_), which begin anteriorly at the edges of the external mouth, and die away in the middle line posteriorly behind a median opening, the atriopore (Fig. 70, _atrp_). From this point a ventral median fin (_vent.f_) extends backwards around the pointed posterior end {115}(caudal fin, _cd.f_), and then forwards along the upper surface (dorsal fin, _dors.f_) to the anterior end of the body. These fins thus constitute a continuous median fold around a great part of the animal (Fig. 70, B, and Fig. 71). [Illustration: FIG. 69.—Amphioxus (_Branchiostoma lanceolatum_) in the Pantano at Messina. (After Willey.)] [Illustration: FIG. 70.—_Branchiostoma lanceolatum_. A, ventral; B, side view of the entire animal. _an_, Anus; _atrp_, atriopore; _cd.f_, caudal fin; _cir_, cirri; _dors.f_, dorsal fin; _dors.f.r_, dorsal fin-rays; _gon_, gonads; _mtpl_, metapleure; _myom_, myomeres; _nch_, notochord; _or.hd_, oral hood; _vent.f_, ventral fin; _vent.f.r_, ventral fin-rays. (After Kirkaldy.)] The surface is soft all over, there being no exoskeleton. The epidermis or ectoderm is formed by a single layer of {116}epithelial cells (see Fig. 72, p. 118), some of which bear sensory processes, while others have a striated cuticular border. There is no general ciliation of the surface in the adult. [Illustration: FIG. 71.—Diagram of the anatomy of Amphioxus. A, anterior; B, posterior part. _an_, Anus; _atr_, atrium; _atr′_, its posterior prolongation; _atrp_, atriopore; _br_, brain; _br.cl_, branchial clefts; _br.f_, brown funnel; _br.sep.1_, primary, _br.sep.2_, secondary branchial lamella; _br.r.1_, primary, _br.r.2_, secondary branchial rod; _caud.f_, caudal fin; _cent.c_, central canal; _cir_, cirri; _coel_, coelom; _dors.f_, dorsal fin; _dors.f.r_, dorsal fin-ray; _en.coe_, cerebral vesicle; _e.sp_, eye-spot; _gon_, gonad; _int_, intestine; _lr_, liver; _mth_, mouth; _myom_, myotomes; _nch_, notochord; _nph_, nephridia; _olf.p_, olfactory pit; _or.f.hd_, oral hood; _ph_, pharynx; _sk_, skeleton of oral hood and cirri (dotted); _sp.cd_, spinal cord; _vent.f_, ventral fin; _vent.f.r_, ventral fin-ray; _vl_, velum; _vl.t_, velar tentacles. (From Parker and Haswell.)]