As a rule the cephalopods swim slowly by the aid of their fins or by a rhythmic contraction by which water is expelled from their siphons, but when in danger the muscular contraction is so violent that they dart through the water with great speed, and even leap into the air to avoid their enemies. But they have another and much more remarkable way of escaping from their foes:--They possess a gland, the duct of which opens into the base of the funnel or siphon, that prepares an inky fluid; and when the animal is disturbed it suddenly ejects this fluid, rendering the surrounding water so cloudy that it is often enabled to retreat unobserved. The 'ink' of the _Sepia_ was used for writing in former times, and is still employed in the preparation of the artist’s pigment that bears the same name. Fishermen are well acquainted with this peculiar characteristic of the animal, for they are frequently bespattered with the contents of the ink bag of the _Sepia_ when the creature is included in the contents of their draw-nets, and have learnt to handle it cautiously until the objectionable fluid has been all discharged. [Illustration: FIG. 187.--EGGS OF _Sepia_] We will conclude this chapter by giving a tabular summary of the classification of the molluscs which will probably be useful to the collector of marine objects. CLASSIFICATION OF THE MOLLUSCA Class =LAMELLIBRANCHIATA=--Plate-gilled. Headless, usually enclosed in bivalve shell. Section =SIPHONIDA=--Mantle lobes more or less united to form tubular siphons. Families--_Pholadidæ_, _Gastrochænidæ_, _Anatinidæ_, _Myacidæ_, _Solenidæ_, _Tellinidæ_, _Mactridæ_, _Veneridæ_, _Cyprinidæ_, _Lucinidæ_, _Cardiadæ_, &c. Section =ASIPHONIDA=--Mantle lobes free or nearly so. No true siphons. Families--_Arcadæ_, _Mytilidæ_, _Aviculidæ_, _Ostreidæ_, &c.