is no contractile heart, and the circulatory system is poorly developed. The Cirripedes are badly furnished with sensory organs; the remains of a simple Nauplius eye may persist, situated on the upper part of the stomach, but the chief sense-organs are the sensory hairs upon the limbs. [Illustration: FIG. 52.—=A=, Dwarf male of _Scalpellum vulgare_, × 27; =B=, diagram of Stalked Barnacle. _a_, Peduncle; _al_, alimentary canal; _b_, brain; _c_, carina; _e_, remains of Nauplius eye; _gl_, cement-gland; _m_, mantle-cavity; _o_, its opening; _ov_, ovary; _p_, penis; _s_, scutum; _t_, testis; _tm_, tergum, seen in =A= as the shaded body above the reference-line of _e_ and to the right of the carina, on the left of the figure. ] The recent Cirripedes fall into six clearly defined Sub-orders. =Sub-Order 1. Pedunculata.= In this division, sometimes combined with the Operculata as THORACICA, owing to the extremely reduced state of the abdomen, the body is borne on a distinct stalk, and the bivalve arrangement of the mantle is clearly retained. The mantle is protected externally by a number of calcareous plates, the arrangement of which is typical of the various genera. It appears that in the most primitive and geologically oldest Cirripedes, the probable ancestors of the Pedunculate and Operculate sub-orders, the arrangement of the plates was somewhat irregular, and they were far more numerous than in the modern forms, so that passing from these older types to modern times we witness a reduction in the number and a greater precision in the arrangement of the skeletal parts. [Illustration: FIG. 53.—=A=, _Turrilepas wrightianus_ (Silurian), × 1; =B=, _Archaeolepas redtenbacheri_ (Jurassic), × 1. _C_, carina; _R_, rostrum; _S_, scutum; _T_, tergum. (After Zittel.) ] One of the most ancient Cirripedes known is _Turrilepas_, which occurs in the Silurian deposits of England, but it is also known from earlier deposits, while undoubted Cirripedes have been found in the Cambrian of North America. The body of _Turrilepas_ is enclosed in imbricating plates, as shown in Fig. 53, A. In _Archaeolepas_ of the Upper Jurassic (Lithographic slates of Bavaria) the arrangement of scutes typical of the Lepadidae is foreshadowed, but the whole of the peduncle is protected by rows of plates (Fig. 53, B), as in _Turrilepas_. The above-mentioned genera did not survive into the Cretaceous period, their places being taken by the genera _Pollicipes_ and _Scalpellum_, which first appeared in the Silurian and persist to the present time, the older and more primitive _Pollicipes_ being represented by about half a dozen living species, while the species of _Scalpellum_ are exceedingly numerous. =Fam. 1. Polyaspidae.=—This family includes the three genera, _Pollicipes_, _Scalpellum_, and _Lithotrya_. [Illustration: FIG. 54.—_Pollicipes mitella_, × 1. (After Darwin.) ]